MyRocks have been archived for MySQL. MyRocks in MariaDB is using a very old version of RocksDB in which is rather buggy. TideSQL is similar to MyRocks but it goes a lot further with it's features, trying to offer parity with InnoDB. TideSQL's core is using TidesDB the library which is a write, and space optimized storage engine written in C from first principles, so from the ground up.
So if Meta stopped maintaining MyRocks, are they moving to a different solution?
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.
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.