Thanks Mathieu, this helps a lot indeed!

The slim service configuration depends on a generated startx, which
depend on this X-wrapper:

--8<---------------cut here---------------start------------->8---
(begin (setenv "XORG_DRI_DRIVER_PATH" (string-append "/gnu/store/8jn365r0kk69j1f4b1q2b7aa2qm1xvg1-mesa-19.3.4" "/lib/dri")) (setenv "XKB_BINDIR" (string-append "/gnu/store/7ar4wyrxbafj7x9ga1ycm5l01q82mw0m-xkbcomp-1.4.3" "/bin")) (let ((X (string-append "/gnu/store/kqibv2arki2dqkl0iczmz6rz0y7ff5vg-xorg-server-1.20.7" "/bin/X"))) (apply execl X X "-xkbdir" (string-append "/gnu/store/m96xy2wyfrg18m638h411k925rg8cjm6-xkeyboard-config-2.29" "/share/X11/xkb") "-config" "/gnu/store/jr40zgab1093hxikp3csp0wnk05v108l-xserver.conf" "-configdir" "/gnu/store/n98412kvx2jyj6k1xqj21iz22gmp0002-xorg.conf.d" (cdr (command-line)))))
--8<---------------cut here---------------end--------------->8---

So it seems that we can't remove mesa from the slim-service closure and
I guess that's expected.

Now xserver.conf drags the Xorg modules, which drags
xf86-input-libinput.

This one depends on libinput, but it also build with libinput-minimal.
Is this a mistake?
I'll test on my system with libinput-minimal and report.

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