I run an ejabberd server and for example the Client State Indicator (XEP-0352), which is one of the extensions that improve battery life for mobile clients, was experimental for a long time but at the same time also available for major servers (e.g. community edition of ejabberd) and mobile clients like Conversations (open source too).
So in my experience, the battery consumption of a modern XMPP client is quite good. When people are complaining about the bad mobile experience they are mostly referring to the time before the mobile extensions were built.
So apparently not everybody shares your definition of 'solved' - I appreciate the alternative opinion, but simply declaring it a myth is, I suspect, unlikely to convince remaining skeptics.
simply declaring it a myth is, I suspect, unlikely to convince remaining skeptics
It's hard to provide specific counter-arguments to anecdotal evidence repeated since back when gtalk was a thing.
[0] https://gultsch.de/xmpp_2016.html
[1] https://wiki.xmpp.org/web/index.php?title=Myths#Myth_Three:_....