Currently just internal errors and that's it.

Option 1: treat unknown content models as 'unknown' or whatever, and allow them to be changed as usual regardless - would be useful for at least getting some usable content out of a broken page, or even just getting the page back, as we cannot do anything with a page with an unregistered content model. Raises potential issues where a content model might have contained private data that should not be surfaced or content that could result in other security issues if changed, as all the raw content will just be dumped on the page this way.

Option 2: treat unknown content models as 'unknown' or whatever, and either don't allow anyone to change them unless they have special permissions (per concerns in the above) or such, or just do something else entirely, such as displaying a message about finding/reinstalling the relevant extension to add it back, providing an option to just delete the page entirely so the title can at least be used again, that sort of thing.

Option 3: ...