Since, 1.0 (or as far as I can remember...), sup would track current until
a few days after a release and then switch to current.  I would "make
build" as files were updated and turn off sup after the release.  I didn't
sup/make build every release, but most of the time this worked well.  The
1.5 release is one that hasn't worked out so well.

Could we have a sup collection of "release15" that today would be the 1.5
sources up until the release and then continue like the sup "release"
collection after 1.5 is released?

The existing "release" collection for 1.4.x would be retired someday.

The "current" collection would continue to point at current.  When
alphas/betas of the 1.6 release are ready, a new collection of "release16"
would be created.

A happy NetBSD user since NetBSD/mac68k 0.9!