When I wrote the tag wiki for engine, I said that a more specific tag is always better but I did mention some cases where it could be appropriate:
Because there are several very different engine types for aircraft,
you should avoid using this tag if you can use a more specific one
like jet-engine or piston-engine.
Use this tag for questions where the specific engine type isn't
important, isn't known or where multiple engine types are being
It looks like none of those cases have really come up yet and of course we can't be sure that they ever will, but I do think there's at least a potential use for it and I suspect that people will keep using it, especially people with little knowledge about aircraft.
But having said that, I think we should focus on actual questions and not on potential ones, so I think it's fine to clean up the existing tags as far as possible and see what happens.