I can imagine that it'd also be barely usable if it was shutting down randomly.
As for new iOS versions making performance worse, that probably more or less has to do with the aging of your battery. I have seen mostly positive reception when it comes to newer iOS versions on older devices.
Regardless, you're complaints are justified after having such a terrible experience. I hope your luck gets better sometime in the future.
No it's not about shutting down randomly, it's about iPhone throttling. Apple was caught doing that and we were lucky to have that class action lawsuit going.
No it's not about aging the battery. It's about the fact that any iOS version can't fix hardware problems - touchscreen failures, white spots and light bleeding.
I owned like, maybe 15-17 Apple devices in the past. Only one never had problems. Just look at it. A single device out of 17! I live in California, so it's not some kind of an isolated experience. And for example my friends' experience (who are living in South America) is much much worse. Fixing iMac problems is just a nightmare, they send it to US and it takes months and months.
New Apple quality sucks. That started around year 2013.