CategoryMembershipChange is great, but it doesn't have a machine-readable way to see if the category has been added or removed from the page. We could parse the comments, but that would rely on reading the comments and matching the message against the translated strings.
I'd like to add some parameters to CategoryMembershipChange::notifyCategorization() and RecentChanges::newForCategorization so that we can update what is in the rc_params db field with information about whether the category was added or removed from the page.
I think this would allow backwards-compatibility without any schema changes.
Adding @Seanchen since he has been working a lot on CategoryWatch and I want to get his input on using this rather than an additional wikitext parse to get category changes.