I do not have the vast Ironman race experience of others. But all the races I have done that logistically could allow a warm-up have had warm ups, including the last WC race. The races that were logistically challenging were New Orleans (very limited access to the water from a dock), Augusta (only access from a temporary doc & downstream), Chattanooga (only access from a small dock & strong current).

River swims and others with restrictive access are very tricky for meaningful warm ups, especially if they are doing continuous TT starts. Those with group starts may be able to pull it off, but they would probably stretch-out the overall start considerably. Not sure that anyone has figured out a grand solution. (Races with easy logistic access to water that do not allow warm-ups have no excuse, IMO. But, I have not witnessed that first-hand.)

Edit: Is no swim warm-up a Covid thing, or just a coincidence of the races that may have been logistically challenged for warm-up.