Aside of the Debian kernel for x86_64, none of the kernel package repos we use provides RT kernel packages, so we’d need to do own kernel builds for every SBC, at least every SoC family, which is very much out of our possibilities with current man power, and it is a too rarely requested feature compared to other tasks on our ToDo. So if you need an RT kernel for RPi, you’d need to compile it yourself. Here are the sources: GitHub - raspberrypi/linux: Kernel source tree for Raspberry Pi-provided kernel builds. Issues unrelated to the linux kernel should be posted on the community forum at https://forums.raspberrypi.com/
I just recognised that there are RT kernel branches for 4.19 and 4.14. Probably there were related packages in the past which are the reason for this topic. But those are not present for current Linux versions. I think it is some build flags which make it suitable for RT applications.