The most drastic, and thoroughly illegal, reaction has been the emergence of Sci-Hub, a kind of global photocopier for scientific papers, set up in 2012 by a Khazak graduate student, which now claims to offer access to every paywalled article published since 2015. source
This subject of the sentence is The most drastic, and thoroughly illegal, reaction, before and the comma is necessary?
Without the comma: Does The most drastic and thoroughly illegal reaction express the original meaning?
Unfortunately, your understanding is not quite correct. To understand the reason for the commas properly, you should know the difference between defining and descriptive qualifying phrases/clauses[1].
Defining qualifying phrases/clauses" are necessary for the understanding of the sentence.
Defining qualifying phrases/clauses are not off-set by commas.
Descriptive qualifying phrases/clauses" are not necessary for the understanding of the sentence.
Descriptive qualifying phrases/clauses are off-set by commas.
The subject is "reaction".
("This" is a determiner and demonstrative adjective. It is required as all singular countable nouns must have a determiner.
Drastic is an attributive adjective and defining - there are many types of reaction - and this tells you exactly which category "reaction" falls into.)
"and thoroughly illegal" is descriptive and not defining. Note that "and" is a conjunction, and thus creates a new clause/phrase.
The commas signify parenthesis - it is as if the part of the sentence between the commas were in brackets:
"The most drastic (and thoroughly illegal), reaction has been the emergence of Sci-Hub..."
Descriptives can be removed and the sentence will still be correct and understandable.
Descriptives are optional information - something that the speaker adds as an aside, i.e. descriptives are extra information that the speaker thinks that you might like to know but which are not truly important.
Imagine I ask you to identify a thief from among 10 suspects and there is only one man wearing a hat...
Compare
The man who is wearing a hat is the thief. Here, who is wearing a hat is definitive and necessary because without that, we do not know which man you are indicating. You will note that there are no commas.
The man who is wearing a hat*, and who lives near to my brother's house,* is the thief. Here , and lives near to my brother's house, is descriptive because that information is irrelevant to me - it might be interesting, but it is not necessary.
[1]there are other names for these qualifiers e.g. defining and non-defining are also used.
The subject of the sentence is reaction. The phrase and thoroughly illegal is simply the interruption clause that should be separated by commas. You can find details and examples here.
Yes, it does, and is more grammatical.
The sentence has many redundant commas, almost to the point that it changes the structure of the sentence. I'd rephrase it as:
The most drastic and thoroughly illegal reaction has been the emergence of Sci-Hub, a kind of global photocopier for scientific papers set up in 2012 by a Khazak graduate student, which now claims to offer access to every paywalled article published since 2015.
