Here’s an answer by co-founder josh about this:
When you delete a data type, it’s not actually deleted yet. It’s just hidden and you can restore it at any time.
If you wanted to permanently delete it, first delete it the normal way, then run a optimization of your app, this deletes unused data fields.
Find it at Settings > General > Scroll down
That’s also great for cleaning up unused CSS settings that build up over time as you are building in Bubble. This increases page load speed significantly.