Summary: These are deliberate choices conveying certain complex social interactions, especially by young people. They are not ungrammatical, nor are they slang. They are also nothing new.
Frame Challenge
First, I’m afraid I’m going to have to issue a frame challenge here
because the question has two conceptual errors in it that must be challenged.
As with spelling, capitalization cannot be “grammatically wrong” because it is about
writing, which is merely technology for encoding actual language, not
the language itself. In the same way, using cursive handwriting could never be considered “grammatically wrong”, nor could writing something using a purple pen. If it’s the same words in the same order, it has the same grammar. How it is written never affects its underlying grammar, which exists in the real language—the spoken one—not the encoding technology du jour.
Strictly speaking, “grammar” includes the twin
key components of syntax and morphology, and more broadly also phonology,
semantics, pragmatics, and discourse analysis. Grammar does not include
style conventions, register, politeness, nor any merely technological
matters such as a language’s orthography ᴀᴋᴀ its writing (ie:
alphabet & script, capitalization, spelling, hyphenation, punctuation)
or its typesetting (ie: font, slant, weight, symbols, glyph variants,
spacing, footnotes, super- & subscripts, colors & decorative ornaments).
Speech is primary; writing is merely secondary, a kind of technological reflection of the primary form of language. For more about this, please see An Introduction to Language by Victoria Fromkin and Grammar for English Language Teachers by Martin Parrott for common misconceptions about writing conventions versus grammatical correctness and various examples of errors in understanding that can arise by mistakenly including spelling as part of the language’s grammar.
“Slang” is part of the language’s lexis, here a restricted kind of vocabulary that’s a type of informal jargon
specifically limited in use by time, place, domain, register, or context; it is not about “approved”
choices of words or expressions. Using peepers to mean eyes or kisser
to mean face is slang because it is using restricted, informal vocabulary, but saying Aincha got no more left? is not slang because there is no informal special vocabulary being used there.
While other definitions and characterizations of those terms can sometimes be found elsewhere, those that I have just now given above are the senses that I mean when I use these terms here.
Some Answers
Unicameral versions of the Latin alphabet have always existed, everything
from the inscription chiselled into Trajan’s Column using only
majuscules (here, Roman square capitals) all the way to the
insular uncial book hand used in the gloriously illuminated Book
of Kells.
Different written contexts in English sometimes require different choices
in letter case. For example, a credit card will always show the
cardholder’s name using only capital letters, while certain computer programming
languages may require that some kinds of names for things be written using
only uppercase in some situations but only lowercase in other situations.
Telegrams were only transmitted in uppercase letters only. None of this is anything that could ever be considered ungrammatical.
The earlier answers that were miswritten as comments tell you why
people may sometimes do this. Consider for example this user’s answer-in-comments:
Multiply the effort needed to move to and depress the shift key
by the number of capital letters you would have to type in a day, and
you've saved perhaps the energy in a few cereal flakes. Over the course
of a year, that might save you a bowlful. This is energy conservation
(or laziness) at its finest--not slang. // Rather than "wrong," I'd
say it's a different style of typing--maybe "ugly" is a better word.
As well as this user’s answer-in-comments:
The lack of capital letters [represents] casual fast speech; very informal,
ephemeral, and destined to last no longer than a month unless that person
is a well-known celebrity. If this was for school work, the author would
fix the errors of capitalisation in a flash, and if they didn't, they
definitely would the next time when they see their assignment scored a
C or D. On an added note, answering "why" questions is basically asking
for guesses and opinions, which is off-topic. The responses might be
interesting but how would you verify them?
I can personally attest to the fact that this particular all-lowercase
style has been perfectly common in all casual computer communications,
even since way back before the creation and general adoption of the Internet
worldwide.
These cannot be considered ungrammatical, of course,
since capitalization has nothing to do with the grammar. It is merely a
deliberate stylist choice made by the typist, just as it was by the poet
Edward Estlin Cummings.
Just ask Doctor Lauren (both of them)
A couple of academic researchers in this area are the linguists Dr Lauren Collister at the University of Pittsburgh, as well as Dr Lauren Fonteyn at the University of Manchester. Both study language use in informal digital contexts and the complex social dynamics we see driving evolving conventions there, including in particular this matter.
From the University of Manchester article cited above about Dr Fonteyn’s work titled “Millennials destroyed the rules of written English – and created something better”, we read:
Fonteyn says that on a superficial level, we can see millennials stripping anything unnecessary from their writing, like the removal of abbreviation markers in "dont," "cant," "im" and in acronyms like tf, ur, bc, idk, and lol. In a world where most of our conversations take place online, millennials are using a number of written devices to convey things that could typically only be communicated by cadence, volume, or even body language.
One such device is "atypical capitalisation," according to Fonteyn, a break from a rule prescribed by standard spelling, which states that capitalisation is "reserved for proper nouns, people, countries, brands, the first person pronoun, and the first word in a new sentence."
"What we see in millennial spelling is different, but not unruly," says Fonteyn. "Capitals are not necessarily used for people (we know who ed sheeran is, it’s Ed Sheeran), or initial words of a text or tweet."
And from Dr Callister in the cited Verge article titled “The way we text says a lot about our personality”, we read:
An interesting thing about texting and tweeting is that it’s an informal way of writing language down. It’s different from other forms like letters, contracts, receipts, or even email in some cases because those are a formal way of writing. So there are conventions: there’s an introductory line where you have to put a comma, you have to put the date in a specific place, you have to use a certain form of language that people generally perceive as very formal. If you don’t use the formal language, it gives maybe the wrong impression. If you use very casual language, if you use slang in a letter or a formal message written down, then it isn’t seen as proper, and we’re taught that in schools — in elementary school, and many of us have been learning this for years and years.
But when you get to text messaging or tweeting, there aren’t really rules for that that we’ve been taught. They are more informal. They are quick. They are intended to be more like those quickly written notes that you pass to your friends in class, or a quick note to yourself, a reminder of something that’s interesting that you might not have a formal way of writing these things down. Without those constraints of structure being very formal, then we have room to play. People can express themselves in a certain style in using different means of language that can show that they’re a little bit different than everybody else. They have their own way of writing, just as we have our own way of speaking to each other.
[...]
Just as when we speak, if we use certain words or phrases, it can indicate where we are from or where we grew up. Using periods or using proper capitalization can indicate that we are perhaps educated or that we are writers or that we are intellectuals or that we value a formal tone in some way. That says something about a person’s style and personality. That’s just another way of being able to express oneself in a kind of different way that might be slightly different from the people around you to be a little bit of an individual.
So these are deliberate choices conveying certain complex social interactions, especially by young people. But this style far antedates the Millennials, as it was common in electronic texts even before the Internet, and even in non-electronic ones like the graffiti found in the ancient city of Pompeii that was destroyed by a volcano in ᴀᴅ 79 nearly two millennia ago, making them an entirely different kind of “millennial”. :)