Hi @vshukla ,
that’s a valid question which also interests me. There’s indeed no official confirmation about the compatibility of the classic editor with WP 5.9.
Maybe you already know, that there’s a plugin test over there -> https://plugintests.com/plugins/wporg/classic-editor/latest
and it seems to indicate that there is no problem, although i can’t assess how broad and realistic this sort of test is regarding use in real life…
