APIs should work, though. So unless the instance needs some kinda captcha or other client-side challenge, e.g. for registration, people could presumably use apps with it.

However, in my experience, not many social-media-adjacent apps support setting a custom proxy, even though modern network libraries should make it a no-brainer. E.g. few Matrix clients support that, and ones that do aren’t much of an eye candy (and have problems with the initial setup of the encryption, which seems to be a pervasive issue with Matrix).