PREEMPT_RT is still a out of tree patch for the kernel. There has been significant progress in mainlining it and for the last few years I've been hearing that its on its last legs of full mainlining.

However, I think its still a little while away from being fully mainlined. The last I remember were issues with preemption in printk calls. Adding this level of preemption support to every subsystem is extremely difficult. Though, I must say that the patched kernels function pretty well. If you need proper real-time, ditch GNU/Linux and go with a real real-time kernel like QNX, VxWorks, etc. Linux will never be true real-time and that's an acknowkedged fact

I only use Debian (or Debian derivatives) and it ships (since at least version 10 of Debian) with a RT kernel which you can install.

It is described as: "This kernel includes the PREEMPT_RT realtime patch set."

So I take it it's still a patch not part of mainline yet?

Out of curiosity I tried it:

https://packages.debian.org/bullseye/linux-image-5.10.0-20-r...

... but only to be met with NVidia drivers telling me they cannot patch real-time kernels!

Didn't look more into it: it's maybe easy to solve.