The software on yax.im has been upgraded from version 0.11 to prosody-trunk. While the new version is still considered as experimental, it provides many under-the-hood improvements that make the trade-off worthwhile. If you notice any issues, please report in the yaxim room (webchat).
The primary reason why this change became necessary is that the previous version of mod_push_appserver (the XEP-0357: Push server for yaxim users) was using synchronous calls to Google FCM to wake up yaxim. This blocked all message processing on yax.im for a brief moment, or for many seconds when there were issues with the network connection to Google’s servers. As asynchronous requests are only sufficiently supported in prosody-trunk, it was required to drop support for older prosody versions in mod_push_appserver, which has happened with the 4.0 release.
On top of that, with the new version it is also possible to further optimize mobile battery usage (mod_cache_c2s_caps) and to upgrade the service to the Lua 5.3 runtime with better server-side memory efficiency (it is currently running on 5.2). These changes will be implemented in the coming days / weeks when the operation has proved as stable otherwise.