Once MCR migration has progressed to no longer reading from the old database schema (see T198557), the following fields can be removed:
and maybe even
Having update.php remove them seems risky, so we'd just leave them until someone manually runs a script. The script should be written to avoid accidental data loss. In particular, it MUST not run (and output a meaningful error message) if $wgMultiContentRevisionSchemaMigrationStage is defined and has the SCHEMA_COMPAT_READ_OLD or SCHEMA_COMPAT_WRITE_OLD flags set (even after this config variable officially no longer exists). It must also present the user with a warning that running this script before the schema migration is complete results in irrecoverable data loss, and prompt for confirmation.