Doesn't work.

My cat who died a couple of years ago at the age of 16 had a bell on his collar all his life. I couldn't pick that bell up without it tinkling but Radar could walk from one end of the house to the other, including the stairs, without a sound. Also when hunting he did most of it by waiting for his prey to get close enough so that any noise from the bell was simply the last sound they heard.

For a while he used to sit on the roof and catch bats

Countering your anecdotal evidence with some of my own: it works fine for many, many cats. Even if the success rate is <100%, it's worth doing.

Can you be sure? I've read before how cats learn to adapt to this very easily. Just because the bell is tingling when you're around and the cat doesn't care, doesn't mean it hinders its hunting.

Or just keep them inside which is 100% successful as well

If your going to keep a cat inside all the time, then you shouldn't own one.

What about un-owned cats? Those are what they say kill the most.

Wouldn't work. Every decent cat collar is designed to come off with only a minimal amount of tugging[1], so even if you managed to get a collar on every feral cat in the city, they'd pretty quickly manage to get the collars off.

[1]: This is behavior that you absolutely want to have in a collar that's designed to go around the neck of an animal who's roaming unsupervised. Consider the (likely) scenario where the collar gets caught on something, and the animal panics. If the collar comes undone easily, the cat escapes from being trapped and saunters off, and you need to buy a new collar when you can't find the old one. If the collar is shut in such a way that the cat can't escape, the cat is likely to injure itself.

I believe maybe it would fall into the jurisdiction of local pet enforcement / RSPCA group to put bells on the cats.

But yeah, that would be no easy task. Trying to herd cats comes to mind :)