I have seen the patch from a muzzleloader smoke and smolder on the ground for 30-60 seconds. I have no doubt that under the correct conditions it could easily start a fire.
I have also started fires with flint and steel and know that it is difficult to do. I can not imagine sparks from a modern forearm starting a fire in any weather condition found on earth.
Years ago on my place in Texas, I had a 100 yard range. The target was a 12”x12”x1.5” thick slab of hardened 4140 steel hung from a steel frame by two short chains. It rang like a bell when hit.
My friend Mitch and I started a grassfire shooting at it with steel core 7.62x39 Russian ammo out of SKSs.
It can most definitely happen with steel core 7.62x39 ammo, which is VERY common.
For sure a public awareness campaign could help reduce fires from shooting. No laws should be considered.
I don’t shoot ML using patch and ball - indeed, it has been years since I even saw someone shoot patch and ball, so I can’t speak to that - although I can imagine how a patch could smolder after the shot.