Fixed:

--8<---------------cut here---------------start------------->8---
(defvar ambrevar/known-mailing-list-archives
  '(("help-guix@gnu.org" . "guix-user")
    ("guix-devel@gnu.org" . "guix-devel")
    ("debbugs.gnu.org" . "guix-bugs"))
  "Alist of mail adresses and their Yhetil name.
Alternatively the key may just be a host name against which a
recipient will be matched.")

(defun ambrevar/guess-yhetil-link (message-id)
  (let* ((all-addresses
          (mapcar #'second
                  (mail-extract-address-components
                   (mapconcat #'identity
                              (list
                               (notmuch-show-get-header :To)
                               (notmuch-show-get-header :Cc))
                              ", ")
                   'all)))
         (mailing-list
          (cdr (seq-find
                (lambda (pair)
                  (let ((address-or-host (car pair)))
                    (if (string-match "@" address-or-host)
                        (member address-or-host all-addresses)
                      (seq-find (lambda (address)
                                  (string-match address-or-host address))
                                all-addresses))))
                ambrevar/known-mailing-list-archives))))
    (when mailing-list
      (concat "https://yhetil.org/"
              mailing-list "/" message-id))))

(defun ambrevar/guess-yhetil-link-prefer-patches (message-id)
  (let ((ambrevar/known-mailing-list-archives
         (copy-alist
          ambrevar/known-mailing-list-archives)))
    (push '("debbugs.gnu.org" . "guix-patches")
          ambrevar/known-mailing-list-archives)
    (ambrevar/guess-yhetil-link message-id)))

(add-to-list 'notmuch-show-stash-mlarchive-link-alist
             (cons "Yhetil" #'ambrevar/guess-yhetil-link))
(add-to-list 'notmuch-show-stash-mlarchive-link-alist
             (cons "Yhetil/patches" #'ambrevar/guess-yhetil-link-prefer-patches))

(setq notmuch-show-stash-mlarchive-link-default "Yhetil")
--8<---------------cut here---------------end--------------->8---

The last line should be useful ;)

Note that I've set two entries, one that prefer guix-patches over
guix-bugs because they seem to use the same recipient domain
(debbugs.gnu.org) and I don't know how to differentiate the two.

--
Pierre Neidhardt
https://ambrevar.xyz/