Keyword being "mint" there. You can still put whatever you want in there, but in order to "register" it, you need to"mint" it by registering it. But you're in no way obligated to register random stuff you put in /.well-known, that'd be bananas to put in a specification like that.
""" A well-known URI is a URI [RFC3986] whose path component begins with the characters "/.well-known/", and whose scheme is "HTTP", "HTTPS", or another scheme that has explicitly been specified to use well- known URIs.
Applications that wish to mint new well-known URIs MUST register them, following the procedures in Section 5.1. """