MediaWiki only supports reading configuration from LocalSettings.php,
which causes problems for a lot of use cases.  For example:

* small local servers would expect `apt install mediawiki-extension-foo`
  to add the appropriate configuration for their wiki
* larger servers (like Debian's own upcoming MW instance) might like to have
  a public config file in a repo and a private file with passwords etc.

It seems like upstream isn't interested in these use cases, so it makes
sense to create a Debian update script in the spirit of `update-exim4.conf`
that manages a configuration directory.

Development work on the new wiki has benefitted from a proof-of-concept
implementation of this idea[0][1] - if you're interested, I'd be happy to
put together a more production-ready version that e.g. doesn't hose the
old version of the file (plus any other requirements).

[0] https://salsa.debian.org/wiki-team/debwiki-dev-
environment/-/blob/main/bin/refresh-LocalSettings
[1] https://salsa.debian.org/wiki-team/debwiki-dev-
environment/-/tree/main/LocalSettings.php.d