The house I live in once had polybutylene pipes. They were replaced with copper several years ago. However, I noticed that they left the pressure reducer valve on the main supply line coming into the house. Is it safe to remove it? I'm assuming that was only need when it was polybutylene pipes, correct? Thanks.

NO! You need it for all plumbing lines unless you are on a well.

For safety purpose I agree with ma0641 it is required for all plumbing, for more information please open the link below, Thanks.

Water Pressure Regulator

In my area of operation, Pressure Reducing Valve is required on all houses where water pressure exceeds 75 PSI. Also, best operating pressure for plumbing fixtures in your house is between 45-55 PSI. I would leave it installed, it won't hurt anything

Milo