I've found that anywhere with rocks and not many people is best. There are some places people meet up but it really depends on the country and the area.

A beach with lots of perfect, round, smooth stones is slinging nirvana. Large bodies of water in general are good but stone quality can vary.

Outside of that it somewhat depends on your level. When I started I learned to throw good distance nearly straightaway, with anything remotely resembling accuracy following much later.

This led to a dangerous phase where I was frequently launching rocks 100m at something like 90 degrees off the intended direction. If you find yourself in a similar position, you basically want to be as isolated as possible and make sure that anywhere your stone can land has a zero percent chance of someone being there.

When you get a bit more accurate you can comfortably sling into the ballpark area with good reliability. At this stage you don't need quite as much isolation so long as you have a large empty area to throw into. Countryside walks are good as you have a constant supply of stones, also, a good field with no stones can be saved by making your own clay ammo.

Slinging is too niche for many people to be doing it, however I've had some success introducing friends to it. Teach by slinging tennis balls between you in a park or something, tennis balls are pretty much safe and don't go as far as solid ammo.

Small stone quarries ,if they're open,and you're allowed to get in, can be quite the spot. Often they have steep walls,as backstop, and plenty of stones, however,usually just not ideal sling stones.