They seem to have stopped during Covid, and never restored the offering. I think CDF» -PMH is the same.
We have "Seat reservations without a seat" on Severn and Solent services. The Inverse (white on black) symbol "R" in timetables means "Seat reservations recommended" which is the case on the 12:45 (Sundays) Westbury to Swindon:
If you book a "seat", however, it come up as seat ** in carriage * ... and when you ask the train manager where to find that seat he says "sit anywhere"
Edit to correct typos