This nonsense seems rife.

A few years ago, my co. was acquired, and they wanted to change our contracts.

New contract had a clause: You cannot do any work - paid or unpaid - without the companies' express permission.

So, like, I can't fire the starting gun at the village 3-legged race without your say-so ?

Cue the usual "oh it's standard we never enforce it" BS. Entire team refused to sign. Eventually it got struck.

They similarly wanted to wheedle in 24/7 support rota into my contract. I simply crossed it out (annotated with a witness signature in the margin) and returned. Never heard about it again.