4.15 removal? Is 4.20 recommended over 4.18?

[Greg Troxel <gdt%lexort.com@localhost>]

I am editing the howto slightly, and that brought up two things that I
didn't have answers for:

  4.15 is going to be removed after the branch, and that has arrived.
  Is this about to happen?

  If someone is going to run xen, should they pick 4.18 or 4.20?   If
  the answer isn't "they should just pick 4.20; of course the more
  recent release is better" then why?

  If running xen 4.20 on any of netbsd 9, 10, 11, current, with amd64
  dom0, and domUs of i3856 and amd64 (with pvshim for i386 pv, as the
  howto says), does anything not work right?