With regard to domestic coal and wood fires, this is a specific issue that has come to the fore again in recent years as having a traditional fire has become a popular experience thing, not as a primary means of heating, and one that because domestic chimneys are in residential areas and quite low down does cause localised high particulates, which is quite nasty if you're asthmatic, for example. So those are much more of an issue than steam locomotives on preserved railways which tend pretty much by definition to be very rural, and also blast the smoke high in the air when running.
Similarly Scout campsites tend to be in the middle of nowhere.
It's important to remember that burning dead dinosaurs/trees causes 2 main issues - carbon and particulates. Most of the action with regard to wood burners and coal fires is about the latter. In essence it's the creation of wider "smokeless zones" where you couldn't burn wet wood or coal in the 80s when everyone had a fire or stove, and solving a problem again that had gone away when everyone got central heating and bricked up their fireplaces, but has come back now it has become popular again as an experience rather than a means of heating your house.