It's NixOS... could they pull just that package from the release-21.05 channel? E.g.,
let
newerPkgs = import (builtins.fetchGit {
name = "release-21.05-pkgs";
url = "https://github.com/NixOS/nixpkgs/";
ref = "refs/heads/release-21.05";
rev = "8d6407e5a442e5e2fc50c3ca36411b6995afbc17";
}) {};
in
and then reference newerPkgs.matrix-synapse in their environment.systemPackages?I may have the syntax slightly wrong, but something like that.
Edit: Ah, except Synapse is a service, so things get a little more complicated. https://nixos.wiki/wiki/FAQ/Pinning_Nixpkgs has some pointers.
I talked to the room admin and apparently when they create a new room (latest synapse, latest element) it gets set to v9 by default and they can't do anything about it.