I was thinking about the realms-aware code, as I have had problems on
  FreeBSD.

Has anyone modified FreeBSD to cope with realms?
The changes as outlined by Jan are very minor.

  > options	 CODA_NOREALMS

  I do not see any good reason to do so. I have run a mix of realms-aware
  and realms-unaware clients, it was not a problem to set up some symlinks
  and choose a realm "compatibility name" to give them the corresponding
  filetree views.

The problem is that the venus-kernel interface has to change.  So the
kernel version (realms or not) has to match the venus version.

  It is not too hard to upgrade both a kernel module and venus at the same
  time, the earlier the better :)
  so why spend developer's precious time maintaining the compatibility mode?

The code is not a module; it is in the base sources of NetBSD.  So if
realms code gets committed there, people have to make the realms
transition whenever they upgrade kernel sources.  IMHO it's better if
these steps can each be taken when desired rather than being coupled.

The code is not much.  Basically, kernel coda version has to be
bumped, a few structures get an extra field, and there are a few extra
comparisons (for same realm).  This is really not a big deal.

A remaining issue is that the coda sources (incorrectly) use their own
coda.h instead of using the header that should be installed by the
kernel code, but isn't.