For some time I advocated some mechanism to convey Compliance Profiles (or whatever name to give them). I agree with @nightpool that they should be used primarily to discover what extensions an endpoint supports. It is not enough to just hyperlink a JSON-LD context document here. In order to learn how to integrate with the extension (message formats, msg exchange, business logic) a pragmatic way would be to include a link to where the extension’s specifcation docs are hosted. These can be in FEDERATION.md, but that is not a universally followed practice, nor does it scale well to complex AP designs (many actors / services, many repo’s / large monorepo’s).
See also: Initiative: ActivityPub "Step On Board" Integration Guide - #19 by aschrijver