Move the .js class logic into a new module, then deprecate it for removal in Drupal 12

Would it be easier to just add a setting instead of a whole module? Similar to how hal had the BC type cast setting in D8/D9.

We could hook_library_info_alter() the js and css supporting this based on a setting, that bit is fine.

The setting would need to default to on for existing sites, so we'd need to treat NULL as TRUE, new sites we could add a default.settings.php entry with it FALSE so it gets written for those. We could have a hook requirements warning if it's NULL or TRUE?

It's doable but I'm not sure it would be less code than a module.

I meant regular config, that's what we've done then. A post update to enable the setting on existing sites, and later on another to remove it again. Thinking about update database dumps and similar, I believe that would be easier to deal with than having to remove it again from those.

Ahh OK config is tricky because we also have to handle config schema/validation etc., we're getting into ✨ Add an API for feature flags Active territory.