My understanding is that Meta continues to use MySQL and specifically MySQL, but have decided they aren't investing in upgrading to the newer MySQL versions. And possibly their fork contains a lot of custom patches beyond just MyRocks. So as a consequence of this decision they no longer support MyRocks for newer versions of MySQL and otoh MySQL no longer supports the 5.x versions.

Not sure at the moment what they are moving to or have moved to. I just heard from Mark Callaghan few months ago about the archiving, not much besides that.