Can a company or employer restrict a person from looking for a job while he/she is put on a PIP?
A PIP is usually a preparation to fire you. So think about it: If you are on a PIP, and you look for and find another job, what's the worst that can happen? They fire you (however, firing you for looking for another job would be something that might be used against them), which they wanted anyway.
Most likely the company is only too happy if you find a job elsewhere and leave on your own.