How do I hide exercises if I don't use them?
Switch Exercises off on Settings → Features. They then disappear from the menu, from the Exercises tab and the form of your sessions and from the tags, and the weight unit disappears from the Appearance & formatting page.
Nothing is deleted: switch them back on and every exercise, block and the exercises on your sessions come back. The switch is available on every plan.
If you picked teaching, coaching or another non-sport profession at signup, Exercises start switched off. If you later move to such a profession, or from one to sport, the app asks whether to hide or show them.