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.