The behavior of external.conf was changed a few months ago.
There is a note on the Upgrade Notes: GitHub - pi-hole/docker-pi-hole: Pi-hole in a docker container (second item).
Also, there are some posts commenting the change, like this: External.conf not being loaded in 2023.1 - #2 by PromoFaux
Your solution is exactly the new recommend way to add changes to lighttpd. 
Just a note:
Any files created directly in conf-enabled will work just like the symlink.
Creating the file in conf-available and adding a symlink to conf-enabled is the pattern used by lighttpd to allow enabling/disabling config files keeping the original files intact (you simply create a symlink to enable a config and unlink to disable it).