We have an open source project that implements settings as content types, where we have implemented a custom content type base/repository using the ContentTypeRepositoryDescriptor pattern. However, these custom content types are no longer visible in the new content type listing in CMS admin in Optimizely CMS 12, nor is the custom content type base available for filtering. I can't see any information about this in the breaking changes document, nor any information in the SDK. I think this is not a very wide used area, so assuming that this might simply be a bug?
The best way I've found to be able to clean up in Content types right now is to look in the table dbo.tblContentType and check the pkID of the content type you want to edit and add that to /EPiServer/EPiServer.Cms.UI.Admin/default#/ContentTypes/edit-content-type/XXXThe page needs a hard reload to load the content type