today I have moved the realtime kernels to [extra]: https://archlinux.org/packages/extra/x86_64/linux-rt/ https://archlinux.org/packages/extra/x86_64/linux-rt-lts/
I have been maintaining these kernels in the AUR via a custom repository  for a couple of years now and believe that quit a few people actually make use of them (in various contexts).
The effort to get the realtime kernel patchset  merged in its entirety has been going on for about 15 years already and the patchset is shrinking with every kernel release. In the future users will be able to configur the kernel build without a separate patchset. That all being as it is, historically there have been two repositories tracking the ongoing work: One for the work on the most recent Linux versions, while the other tracks all LTS branches that are supported. I am intending to follow both the most recent and the highest LTS version, while consolidating with core/linux and core/linux-lts wherever it makes sense.
For that purpose we have moved the kernel repositories (with distribution specific patches), mirroring their respective upstream to:
This allows for easy collaboration and cherry-picking of patches (which I do currently already for linux-rt, using core/linux) and my hopes are, that the knowlegde in our developer and packager team grows in how to configure kernel builds and maintain the packages.
The https://gitlab.archlinux.org/archlinux/packaging/upstream/ group location is meant for all projects that have to maintain custom patches on top of what upstream does (that is also why shadow lives there currently).
In case there are developers or package maintainers interested in collaborating on the builds and maintenance of the kernels, please reach out!