I think this is all very airline specific. I’ve flown on airlines as a passenger where there is absolutely no reaction whatsoever from the crew when passengers stand up during taxi (or even during landing!). On the other hand, at my place of work the cabin crew are very proactive and can be heard on the PA as soon as someone even thinks about unbuckling their seatbelts. As a consequence there is almost zero need for us pilots to even consider stopping the aircraft, the cabin crew have it well in hand. If there’s a delay during taxi we will often make a PA from the cockpit explaining that we have not reached our parking position yet, backing up our cabin crew in their effort to keep everyone seated.
So it all seems to depend on the cabin crew assertiveness and the airline culture and policy.