The latest Debian-compatile unison version in portage has been 2.48.15_p4-r2. At the moment the minimum version is 2.51.3_p20201127, so unison synchronization with Debian Buster systems (or any other Linux distribution using <2.51 versions) is gone.
We could have delayed the removal of this unison version at least until Debian 11 (Bullseye) is released at some point in summer, which ships unison-2.51.3+1 and ocaml-4.11.1-4. (Note that unison relies on ocaml marshalling for transport.)
Author: Volkmar W. Pogatzki <email@example.com>
Date: Fri Apr 16 08:46:28 2021 +0200
net-misc/unison: remove :2.48
Package-Manager: Portage-3.0.17, Repoman-3.0.2
Signed-off-by: Volkmar W. Pogatzki <firstname.lastname@example.org>
Signed-off-by: Joonas Niilola <email@example.com>
Perhaps it would be better to file a bug at debian.
Or should we really add a masked version of the old package?
1. Unison is a "cross-platform file synchronizer".
2. Unison uses ocaml marshaling, which keeps changing, so it is very sensitive for matching versions.
In my opinion, these two points lead to an obligation for some long-term planing towards the question which platforms are to be supported as sync partner. Distributions with a standard release model, like Debian, are quite clear on this, e.g., Buster ships unison-2.48, Bulleseye ships unison-2.51.
Sure, I understand the difficulty of maintaining old unison versions and, in particular, the ocaml dependencies. However, unison-2.48 has been around for quite some time now, and some three more months would not hurt too much, I guess.
Why has it been removed now, why not 3 months earlier, why not 3 months later? Keeping track of other platform's versions would be a reasonable decision basis in my opinion. This is why I suggest to keep the 2.48 version for another three months or so, when Bullseye has been released (for some weeks or so).