MIX is interesting. Here's the introduction.
ejabberd seems to support MIX, maybe its better to look deeper on its implementation before enabling it.
So far Converstations has an issue but no work is documented there, nothing for Gajim. We'd be pratically enabling the XEP for... the Swift client..?
This is really more of a suggestion or unified tracking of what happens to MIX development. The spec seems not dead, it got updated last month(Version 0.14.0, 2018-12-10).