I agree to a certain extent, but I don't think you need to support everything, just the subset of features that a particular user cares about. We should have so

I agree to a certain extent, but I don't think you need to support everything, just the subset of features that a particular user cares about. We should have some form of baseline that's more than the basic protocol, and the Compliance Suites will hopefully help there, but in general as long as basic chat works it should be fine. The first party clients can support everything and give you the best experience, the third party ones are juts in case you can't run the first party clients or need eg. better accessibility support or similar. For example, I used to use Mcabber with HipChat at work. Mcabber doesn't support even basic modern XMPP things (history, for example), but it was plenty good enough for me to chat with my coworkers.