The only thing that seems relative new here is usage to refer to a specific known individual, of known gender. Historically singular they was used definite but unknown individuals, or at least individuals of unknown gender.
In any case, "They is" is wrong for singular, just like "you is".
"You" always takes "are", for both singular and plural "you". Indeed singular "you" differs from plural "you" only in the reflexive: yourself/yourselves.
Singular "They" is exactly the same. It uses "are" and differs only in the reflexive: themself/themselves. The only difference here is that the plural reflexive can be used for the singular, and indeed that has been standard for several hundred years, but feels extremely awkward with respect to a known specific individual.
Fun fact: "themself" actually predates "themselves", and was originally used for both singular and plural they, although around 1450 "themselves" became standard for plural, with "themself" remaining standard for singular. But eventually "themself" almost completely died out.
Furthermore, "He" has been used as a generic pronoun long predating the singular they. While it comes with some ambiguity as well, it flows a lot better and does not screw around with singular/plural ambiguity.