Is there an ETA for an updated version of the migration plugin that lets us switch to CP from WP 5.1? Alternatively, can anyone confirm whether the workaround filter or other workarounds are safe to use in conjunction with WP 5.1?

The new version of the migration plugin is out now: Release Version 1.5.2 · ClassicPress/ClassicPress-Migration-Plugin · GitHub It supports switching from WP 5.1 and it switches to ClassicPress 1.0.0.