Can we use "who" by itself as a subject in spoken language?

Question

For example, I want to say " Students who didn't come here yesterday are lazy." But I say " Who didn't come here yesterday are lazy" instead because I talk to students and I don't want to say "students" again. Does it make sense?

Answer 1

This is called a "headless relative clause" or "fused relative clause". In English, the pronouns who and whom cannot normally be used as fused relative pronouns:

The categories 'person' or 'people' are [...] so general in meaning that they normally take a further qualification, as in The people who came were all enthusiastic. [...] the head noun people is almost redundant; still, it cannot be left out in English, i.e. we cannot speak of *Who came were all enthusiastic. Obviously humans are too important to be left unmentioned as a head. The grammatical solution English has found is using relative clauses headed by the demonstrative pronoun those for plural humans (25a) and the quantifier anyone (25b) or the personal pronoun he (25c) as heads for singular humans:

25. a. Those who say so are liars.
    b. Anyone who says so is a liar.
    c. He who says so is a liar.

(Cognitive English Grammar, by Günter Radden, René Dirven, p. 162)

So it is not correct. You need a noun or pronoun before "who".

In this case, the pronouns that would work are "those" and "anyone".

1. "Those who didn't come here yesterday are lazy."
   You can say this if you know some people didn't come.

2. "Anyone who didn't come here yesterday is lazy."
   You should say this if you're not sure if there are some people who didn't come.

As MorganFR and 1006a said in comments,

3. "Whoever didn't come here yesterday is lazy"

is also possible.

Answer 2

Yes you can avoid saying 'students' again, when your 'who' follows 'those' ( here a pronoun indicating to whom the who is pointing to)

Answer 3

Who didn't come here yesterday are lazy is fine if and only if you want to employ archaic or otherwise esoteric English. Otherwise, as sumelic explains, the construction is not used in contemporary English. But it was once and even now shows up in limited contexts. Consider

-Who rides at the tail of a Border thief, he sits not long at his meat.
Rudyard Kipling, 1892

-Who steales my purse, steals trash.
Wm Shakespeare, Othello

The Oxford English Dictionary cites uses as far back as the 13th century.

More recently

Who dares wins, (who hesistates is lost) has become a slogan and catch phrase. It's based on a word for word translation of the Latin qui audet adipiscitur.

The slogan is used by the British Special Forces (S.A.S.) and as such is attributed to S.A.S. founder Sir David Stirling (1915-90), one of whose biographies is titled Who dares wins. See also The New York Times:

Colonel Stirling's exploits were recounted in a book by Virginia Cowles, "The Phantom Major" (Harpers, 1958). The group he had formed became the S.A.S., a cloak-and-dagger operation that was recently employed, with some controversy, against the Irish Republican Army. Its motto: "Who Dares Wins."

(Sir David Stirling, 74, the Founder Of Britain's Elite Commando Unit)

The slogan has taken a life of its own and appears in many places, including songs, computer games, a 1982 UK film title, a 2015-present British "reality TV" show, and blogs of regular people. (Most of these uses fall within the sphere of British English, not American English.)

Wikipedia uses Who Hesitates is Lost as a poorly translated title of the 1960 Italian film Chi si ferma è perduto. I say 'poorly translated' because in contemporary English, as sumelic explains, native speakers don't use '“who” by itself as a subject in spoken language'--or written language. But it is not illegal to so, and you can cite Shakespeare and Kipling if someone challenges your usage.