It's up to the admins of the instances to delete them, but I think it would make sense to delete them.

There was a lot of communities opened a few years when the first wave of redditors wanted to move everything here. It has a negative effect when people search for a topic and find empty pages and no activity, as it appears as if there's no interest in those topics. It would be better if it was completely empty, so anyone with legitimate interests would feel more inclined to start new ones.