Once upon a time, the word "you" was plural, or formal, like "vous" in French. "Thou" was the everyday singular word, though it didn't go with "is", but that's another story. Over time, "you" gained a singular meaning but the verbs did not change. Consider:
You are asking about what verbs to use with singular they
That sentence has "are" in it. But it is singular you. Did you even notice until I pointed it out?
Stop thinking that "are" is a plural verb. In "you are asking" it is a singular verb. A normal and everyday one, for that matter. And so in "I am worried about Chris. They are really struggling in school" it is also a singular verb. It just is. Do not write "they is" -- it is as wrong as "you is" and that's why you're seeing it flagged by your tools.
[Random arguments about it being confusing if you just use a "they are" sentence out of the blue are of no interest to me. Precisely the same argument applies to not knowing who "he" refers to if there was no previous mention of a person, or of not knowing if "you" means just one person or a group. Further, in English "we" is ambiguous about whether it includes the person being spoken to or not. All this ambiguity is part of pronouns and we deal with it. This question is about whether "they are" is plural or singular and comments bringing up ambiguity belong somewhere else.]
Grammarly displays as wrong
– Coder2195 May 12 '21 at 15:58