Mountain Lion went a long way towards fixing Lion's problems, and Mavericks just about finished the job. Which is why I run Mavericks. The only remaining Lion things I really dislike are the Launchpad, the hidden Library folder, and some minor-ish aesthetic differences. I've patched some of these.
Hm. I've never written anything serious for Apple platforms but of course played around for a bit. But, I've always assumed ARC is implemented purely in the compiler, isn't it? I remember disassembling something I wrote and learning that the compiler inserted retain/release calls as necessary.
> Which is why I run Mavericks.
Actually, I ran Mavericks for several years after it was superseded. I was made fun of by some people (who complained about glitchy WiFi on Yosemite, lol). Had to finally update when I got a new job and needed to compile an iOS app, which required latest Xcode, which required latest macOS. Then I stayed on Mojave for like 2 more years, refusing to update to Catalina to keep using 32-bit apps. And then several months ago I bought an M1 Max MacBook, which means Monterey.
ARC does partially work on Snow Leopard, but only for 64-bit, and it's limited (namely, you can't use weak references).