It's been policy for years! In almost 8 years I have only been refused once out of about 60-90 BA sectors and that was pre-Covid out of GOA to LGW which is non-BA check-in staff and the policy had only fairly recently been introduced. One refusal out of about 60-90 European sectors and simply due to poor internal communication by BA to outstations.

There used to be signs up (might still be) at T5 and LGW South in the check-in areas even informing all pax that their hand-baggage could be checked-in FOC. It clearly helps BA with the boarding process by reducing the amount of carry-on at the gate.

Just to add, I never buy anything but an HBO fare although I used to occasionally to a POUG for a sector, maybe 1 in 10 sectors.