cc @vcunat who committed the removal.

Builds fine with this revert.

The entire commit should be reverted because we really shouldn't remove packages on the stable branch. For example, it breaks

boot.kernelPackages = pkgs.linuxPackages_testing;

• I consider it good to remove unmaintained explicitly versioned kernel branches. Those explicit attributes are just an artifact of supporting _latest on 18.03; I can't see what to improve about this workflow.
• linux_testing (at 4.16-rc4) was removed halfway (by me) and that's certainly bad. For now I made it an alias of _latest 5e10df3. I don't expect people will backport -RC bumps to 18.03, but if they do, we can point it to them.
• This MR: on a superficial look it appeared to be some 4.15 kernel, and I assumed those are unmaintained upstream (i.e. possibly vulnerable).

I am quite out of the loop on whatever the kernel support cycle is and entails, but that's quite a regression to me, and the presented reasoning doesn't impress me in the slightest.

Yes, bcachefs is currently only present for 4.15. Yes, that version may be unmaintained and, in general, bcachefs is quite a non-standard little tested feature. But still, it was in stable NixOS for quite some time, always lagging behind a bit. No, dropping it out of nowhere because you found out it lags behind once again is not acceptable. We know what we are doing, thank you. We don't really have an option other than to use it on 4.15. Now bring it back, as we have no other option of using it. If you hold something substantial against shipping it in stable, let's discuss it and consider dropping it for the next release, but not now.