MediaWiki 1.32 should support MCR, but still remain backwards compatible. This allows wiki instances to back out of MCR without losing data, and preserves compatibility with any extensions or tools that make direct use of the old schema - specifically, of rev_text_id, rev_content_model, and rev_content_format.
The new default should be:
$wgMultiContentRevisionSchemaMigrationStage = SCHEMA_COMPAT_WRITE_BOTH | SCHEMA_COMPAT_READ_NEW;
With this setting enabled, update.php should convert to the new schema, if that did not previously happen. If update.php detects $wgMultiContentRevisionSchemaMigrationStage = SCHEMA_COMPAT_WRITE_BOTH | SCHEMA_COMPAT_READ_OLD; it should also perform the conversion, if still needed, and issue a warning that $wgMultiContentRevisionSchemaMigrationStage should be changed to ensure forward-compatibility with the next release. If update.php encounters $wgMultiContentRevisionSchemaMigrationStage = SCHEMA_COMPAT_OLD, it should warn that the setting should be changed and update.php should be re-run after that change.
NOTE $wgMultiContentRevisionSchemaMigrationStage will only have a pre-existing value on wikis that were installed off a snapshot or master, since $wgMultiContentRevisionSchemaMigrationStage did not exist in 1.31.
NOTE: This enables the usage of extra slots. If this new ability is used, and the wiki is then set to only reading the legacy schema, access to any content in the extra slots will be lost.