Suzanne, TR3 is made for protecting car paint so I doubt it will harm the paint on a sewing machine, but you are right to be cautious in using it on a very old machine.
Dianne, I have a family member who owns an auto body shop, I'll ask him why TR3 now has that warning. Something must have changed, either the formula for the TR3 or maybe there's been a change in how cars are painted or how clear coats are made.
Cari