We don't need BE to forbid it, just want a way to remove it from the PleromaFE for special use case instances. As much as the greater community loves Subjects/CWs, it's a really bad UX that doesn't fit with every use case of Pleroma (e.g., private/unfederated).
#147 (closed) is a big issue, see third checkbox there - it's about CW/Subject.
I'll explain: there's little point in completely disabling subject field completely (as well as scope options) since backend doesn't care and user can easily bypass FE's restrictions by using mastofe or mobile clients. Hiding it would also be bad experience because user loses control of it when replying to CW'd posts.
My proposed solution is to hide it if it's empty (with a user/instance option to keep it visible always). This way you could have it hidden away by default and when replying if you have subject copying set to "none", but if you have subject copying you will have that subject field visible when replying to someone and see what exactly you're going to post.
If you're gonna do it, I'd pretty much want it be done that way, but I'm open for discussion.