On Friday, June 11, 2021 at 10:01:23 AM UTC-4, jane doe wrote:
The Uniform Plumbing Code and International Building Code, in accordance with ASSE 1016, require that all tub/shower faucets now be of the pressure balance or the thermostatic mixing valve type. Two and three valve handles are not. The purpose for this is to prevent serious harm from hot water scalding.
The exact text from the Uniform Plumbing Code book, ISSN 0733-2335, states in section 420.0 €“ SHOWER
AND TUB/SHOWER COMBINATION CONTROL VALVES:

€œShowers and tub/shower combinations in all buildings shall be provided with individual control valves of the pressure balance or the thermostatic mixing valve type. Gang showers, when supplied with a single temperature controlled water supply pipe, may be controlled by a master thermostatic mixing valve in lieu of individually controlled pressure balance or thermostatic mixing valves.€� (pg. 30-31).

So what does this all mean to you? Well, if you have a 2 or 3 handle tub/shower valve currently in your home, you have been grandfathered in and you dont need to immediately replace it. However, if something goes wrong with that valve and it must be replaced, you will have to do so with a single handle pressure balanced or thermostatically controlled valve.

Sounds dubious to me. I can see it as a requirement if the shower/tub is being replaced,
it's a bathroom remodel, etc. But not if it's just that the old valve needs to be replaced.
And whatever it is, it's always up to the local or state authority that has jurisdiction as
to if it's OK or not.