If you hit an explicit limit, I'd expect the thread spawn to fail, and most processes to panic if thread spawning failed, sure.

But if you run below the explicit limit, but above the implicit limit of whatever continues to work, it's not surprising to me that the OS just freaked out.

You could report it to Apple, but their reporting process is pretty opaque, especially if you're not a registered developer (because why would you be, if you're just using mac os because you like the desktop environment, or whatever). Who knows if they'll fix it, but it's not worth making a big deal over, because you weren't really wanting to run 2k threads on mac os anyway, because it would suck, even if it did work.

From the other message on the thread; it looks like too many threads is causing a watchdog timer to fail, leading to the panic.