I bred some very difficult rainforest dwarf Cichlids and killies in both snowmelt and rainwater - within view of the highrises in my city. If I were "just" keeping fish because I liked them, I wouldn't do that - but I enjoy breeding difficult fish.
I did no special filtering. I waited to collect rain til I figured the bird poop had washed off the roof.
The system wasn't good because very soft water is unstable, and so a regular water change schedule is needed. If it didn't rain for a few weeks, yikes. It was also physical work to carry in enough to do the job. Water's heavy.
I also suspect the wind directions, etc, could have mattered. In my new set up I will have sea breezes, and I'm not sure if salt air would be an issue. Hmmm. you never stop learning in this hobby.
Once I got an RO unit, all I needed was a reservoir and a sink that could be used for that slow steady process, and life got easier.
I moved and discovered the tapwater was soft in the new house, and never looked back.
In general, I agree you let your water source decide what fish you can keep. If you really get into breeding, things change, but for day to day
Ps I sent this yesterday but the site update seems to have put it on hold, so I hit send this morning anyway.