The last stable opendkim release is from 2015 and does not handle some
signatures such as ed25519 which was standardized for DKIM in 2018.
Both Fedora and Debian ship with 2.11.0 alphas/betas in their stable
releases. Since the development branch has not seen any new commits
since 2018, I doubt there will be a new stable release any time soon.

Motivation for this change

I noticed that the DKIM signatures of some incoming mails were not parsed correctly. For example this valid DKIM ed25519-sha256 signature resulted in an error:

DKIM-Signature: a=ed25519-sha256; bh=qNcvQm/9txhkK90cpARQSVovodq1gvD8UviIMqIHfrs=;
 c=relaxed/relaxed; d=ctu.cx;
 h=Subject:Subject:Sender:To:To:Cc:From:From:Date:Date:MIME-Version:MIME-Version:Content-Type:Content-Type:Content-Transfer-Encoding:Content-Transfer-Encoding:Reply-To:In-Reply-To:Message-Id:Message-Id:References:Autocrypt:Openpgp;
 i=@ctu.cx; s=default; t=1611224151; v=1; x=1611656151;
 b=KKlLTXsa+JoY7FaUbrYan02pc9bnpgOfOclKriBgg1iQpKcyxGWboVdFVsX+kw0EE2KMvLq+
 L5ct0hUn4CMIDg==

	dkim=permerror (0-bit key) header.d=ctu.cx header.i=@ctu.cx header.b=KKlLTXsa

So I read a bit on the mailing list and found a hint that the latest development releases support this. I was aware that another mail server running with opendkim on Debian was able to parse the DKIM signature correctly, so I looked at their packaging.
With the latest beta release (from 2018) it parses the header correctly:

dkim=pass header.d=ctu.cx header.i=@ctu.cx header.a=ed25519-sha256 header.s=default header.b=P6dpwuS9

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.