If you consider these releases "official", maybe, at least as far as "desktop operating systems" go. It's older than Windows; Mac OS is older, but you could argue that modern-day Mac OS isn't really descended from classic Mac OS, so it doesn't count.

Sibling comment points out that various still-alive Unixes are, in a sense, still the same operating system as the original Unix.

I suspect various mainframe operating systems are older and still maintained, but that's a little out of my wheelhouse.

System V Unix was released in 1983 and I'm pretty sure Solaris, AIX, and maybe some other still-maintained Unix variants are still based on that same source code.

EmuTOS is older I believe. Or at least just as old. GPL'd version of the Atari ST operating system. It's directly descended from the GEM/GEMDOS sources from Digital Research dating back to 1983 if not earlier. And I believe there's pieces in there that go back further -- stuff that came from e.g. CP/M 68k.

2.11BSD still gets maintenance fixes when problems are found and of course you can trace a chain of descent for it all the way back to the first versions of Unix which were installed at Berkeley.

In terms of operating systems that mere mortals would have access too, quite possibly, though IBM may beg to differ (along with other companies that have inherited mainframe clients and IP).

It may be limited, but I think there is still work done on CP/M.

QNX is early 80s as well.