I thought I’d make a post here to increase visibility of an Issue that has been raised bout the CP Migration plugin.
The plugin is in need of some testing on more recent version of WordPress. WP 5.8 is due soon and as it currently stands the migration plugin only officially supports migrating from version 4.9.0 to 5.5.3.
The Issue linked above is actually, as far as I can tell, more about WordPress themes, rather than the core itself.
So, the issue seems to be linked more to the Twenty Twenty One theme.
As a way forward should we consider checking for themes in the migration plugin that have known issues and encouraging a theme switch before migration, or do we back-port these functions - bearing in mind that the latter approach is likely to get more heavily linked to the Block Editor with time.