filters.CHAT doesn't really have a purpose as Message.chat is not optional. I would assume that filters.CHAT was added in #2759 as constant version of filters.Chat and that this is superfluous was just missed in that rather large rework.

We should deprecate that filter. Since issuing warnings on filter access is non-trivial, this would be a documentation-only deprecation but should still get its own commit for more visibility in the changelog.

Reference: https://t.me/pythontelegrambottalk/248839