It can be weird though. I had a hobby I kept meaning to get back to Some Day(tm) and when I did the app was on my device just like always, but I was now two devices later and it would simply crash on startup.

I’m fairly sure that at some point those devices ran low on space (less of a problem today, but mostly because I’ve been off the music treadmill for a while) and I had to delete things to keep working.

Yeah, you do. Apps routinely disappear from the AppStore, e.g. after some M&A and such. They just vanish from your purchase history without a trace. No way to get a copy if you don't have one already.