Yes, that's more 'keeping the option open' than 'we do this regularly', and I can't seem to find any documentation that MySQL gives any better guarantee. Note

Yes, that's more 'keeping the option open' than 'we do this regularly', and I can't seem to find any documentation that MySQL gives any better guarantee. Note that for all of 10, 11, 12, 13 and 14 no changes have been made in the storage format of tables that made it mandatory to rewrite any user-defined schema. I will admit that some changes have been made that make the data not forward-compatible in those versions; but MySQL seems to do that at the minor release level instead of the major release level. MySQL 8 doen't even support minor release downgrades; under PostgreSQL this works just fine.