See the conversation in https://github.com/ircv3/ircv3.github.io/pull/285 for some background on this. I suspect most of the work for this is done, but jwheare points out a few good things:

At minimum you'd need to also handle the TAGMSG command, and client-only tags (prefixed with +) in order to request that cap, and make sure you're aware of the changed size limits on the tag space ircv3.net/specs/extensions/message-tags.html#size-limit

Hey freaktechnik -- do you think there's any use for us to handle TAGMSG? Or just doing nothing with it is probably reasonable until there's a tag to support? It looks like we already handle client-only tags (although I've added a test for it).

I don't know if the size limits matter since we don't enforce those on our side anyway.

It seems there is a non-draft spec that uses tagmsgs that woukd even be applicabke to the prpl chat interfaces: typing notifications. https://ircv3.net/specs/client-tags/typing

Also, client side tags are only supported for incoming messages, I don't think we have any infrastructure for sending tags, currently.