In many US states, provisions in any contract forbidding you to take certain other jobs are entirely unenforceable. In some states it is illegal even to ask you to sign such a contract (although many employers do anyway), but in most others where they are unenforceable, employers can still ask and hope you are fooled.

There are probably whole countries that forbid these contract provisions, or forbid enforcing them.

Don't assume that any particular right you signed away to get a job is really gone. Ask a local lawyer. Expect to be surprised. In many cases, even when there is no specific law, case law has interpreted such clauses very restrictively, making them de facto unenforceable for, e.g., anybody below company officer level.