Earlier quoted context omitted.
Well, extending the spec could be the solution too. What I was trying to convey was more that there will be a lack of such clients until there is either an agreed-upon extension to the standard, or a popular enough fork of the standard. (at least as far as XMPP is concerned)
There is an XMPP extension for it: http://xmpp.org/extensions/xep-0136.html The problem is, there are far too damn many XMPP extensions and too few of them see any kind of real adoption.
XEP-0313 is a modern replacement, focusing on just the things that people actually need. It's implemented in many servers and clients already (see https://www.zash.se/mam.html ). It's still under development, but it's already in active use by many people.