The previous state of the relation was:

name=Folly Bridge Bus Gate
source=https://www.oxfordshire.gov.uk/residents/roads-and-transport/parking/parking-and-bus-gate-fines/bus-lanes-and-bus-gates
type=restriction
restriction=only_left_turn
except=psv;bicycle

The "except" key is documented on the wiki as the way to do what you are pointing at. "psv" includes buses. Was it not working?