After applying:

diff --git a/gnu/packages/lua.scm b/gnu/packages/lua.scm
index edb3f85109..36fd1eb066 100644
--- a/gnu/packages/lua.scm
+++ b/gnu/packages/lua.scm
@@ -1175,6 +1175,8 @@ enabled.")
               (snippet
                '(begin
                   (delete-file "fennelview.lua") #t))))
+    (properties
+     '((release-monitoring-url . "https://fennel-lang.org/downloads/")
))
     (build-system gnu-build-system)
     (arguments
      '(#:make-flags (list (string-append "PREFIX=" (assoc-ref %outputs
"out")))

$ ./pre-inst-env guix refresh fennel
gnu/packages/lua.scm:1162:2: warning: no updater for fennel

It should otherwise clearly work since 
https://fennel-lang.org/downloads/ is a classic directory listing like
the generic-html updater should support.

I think it may be because this package uses a git-reference rather than
url-fetch.