Does removing the plate and screws from a sliding genioplasty compromise the mentalis muscle?

There is no harm in keeping titanium hardware in your body for your lifetime. There are no documented problems with such retained hardware. Every time you open up the chin you are separating the attachment of the mentalis muscle which then has to be put back together. The less you reinjure the mentalis muscle the better. If the only purpose of doing so is to remove the hardware I wouldn't do it for that purpose alone. But if other procedures need to be done, then you have little choice.