Particulates are one reason most city buses in the United States now run on natural gas. That's a real improvement over diesel in terms of particulates.
Honda used to sell a natural gas civic and you could buy a home fill station, in the late 2000s it probably made a lot sense. But now, you'd be better off getting an electric vehicle and a home charger.
I think LPG is interesting, but if you're going to deal with speciality fuel/fueling, you can just get a hydrogen or battery electric vehicle and enjoy the benefits of an electrified powertrain. LPG powertrains retain the complexity and service costs of gas powertrains with pretty marginal benefits. Electrification has its own costs but unique advantages.
Not universally true. In Italy, where regular gas prices are stellar, LPG is a relatively common thing. Not common enough, but the infrastructure is "there" (although not as capillary as regular gas) and it's advertised as a consumer option.
When oil became cheap again, the government stopped the subsidies and it was back to petrol. The tanks and the pumps at the services stations were ripped out and sold to the Indians, Eastern Europeans and the Chinese.