I grew tired of the RoR’s “we work on any database” as it is mostly more pain than gain in the long term. So I would be in favor of dropping the support for everything but PostgreSQL.
I would say MySQL is not such painpoint as SQLite is though, as for MySQL we always can find a workaround, just need an if statement, but the SQLite is just missing a lot of capabilities.
Having the RDBM requirements stated is a good first step though! We can always change it there to state dropped MySQL support 