I thought that you can still sell new props that need leaded fuel, is that not the case?

Note, since the 80s the vast, vast majority of piston driven aircraft engines have been able to operate on unleaded fuel. We know this because for decades GA pilots have been filling out the paperwork for an experimental fuel variance and then running these engines unmodified on the cheaper unleaded they got from the gas station down the street without any apparent issue or rise in engine maintenance/failures among pilots that do this. The main hurdles being the necessary and not insignificant paperwork as well as concern over insurance rates.

From my understanding there was a problem with one series of engine in the seventies that was suspected to be due to unleaded fuel among the more modern product line of a major manufacturer, and while the engine was modified to fix it neither Lycoming nor Continental, the two primary piston engine manufacturers who make up the vast majority of the market, saw significant pressure to drop the official recommendation for unleaded until relatively recently.

Since the US finally started to get serious about phasing out leaded avgas in the 2010s, and the aditude of its been fine so far so why risk any change has run up against said pressure, both have to my knowledge dropped the requirement retroactively with no modification necessary for the majority of their historical and current product line.

You might need to re-engine or more likely just get an exemption for flying history aircraft, but the benefit to the hundreds of thousands that live near GA airports in terms of reduced damage to children’s nervous systems far outweighs the nebulous cost of switching the default form of avgas.