More complete list: https://www.mediawiki.org/wiki/Extension_default_namespaces

TimedMediaHandler and ProofreadPages defined namespace that has a configurable IDs, but the default one is commonly used and there is plan to harmonize them (T123823 and T74525). The default namespace number is not used by anything else, so it is still good to add.

Wikibase (which is not even listed in the description) also uses configurable namespace names for its Item and Property namespaces, and even namespaces not designed to be configurable can be configured by define()ing the namespace ID before loading the extension. Couldn’t the message be specified in extension.json (for not-configurable-by-design namespaces) / a hook (for configurable-by-design ones) instead?

Change #1065478 had a related patch set uploaded (by 01tonythomas; author: 01tonythomas):

[mediawiki/extensions/Newsletter@master] Add namespace description for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1065478

In T373070#10089423, @Tacsipacsi wrote:

Couldn’t the message be specified in extension.json (for not-configurable-by-design namespaces) / a hook (for configurable-by-design ones) instead?

It could, that would be a logical next step to support describing them on this page. It would however take some work to define that property and that hook, and then shuttle the data over to where it's needed on Special:NamespaceInfo, and I'm not really sure if anyone will care enough to use that property/hook. Patches are welcome if you want to add that, I'd review them (let's maybe do this in a separate task).

even namespaces not designed to be configurable can be configured by define()ing the namespace ID before loading the extension

(that's a little bit horrible, I had no idea you could do that)

In T373070#10089584, @matmarex wrote:

Patches are welcome if you want to add that, I'd review them (let's maybe do this in a separate task).

I may do it if/when I have time and mood for that (no promises!).

(that's a little bit horrible, I had no idea you could do that)

It actually makes sense: there can be an infinite number of extensions developed for MediaWiki, but only a finite number of namespace numbers exist, so it’s possible that a wiki administrator installs two extensions with conflicting namespace numbers. If at least one of the extensions handles namespaces as it’s supposed to do, by always using the NS_* constant (never hardcoding the namespace number), the wiki administrator can resolve the conflict by changing the constant. (It was introduced in T160462: Allow namespace IDs defined in extension.json to be overwritten in LocalSettings.php.)

Change #1065478 merged by jenkins-bot:

[mediawiki/extensions/Newsletter@master] Add namespace description for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1065478

Change #1080256 had a related patch set uploaded (by Msz2001; author: Msz2001):

[mediawiki/extensions/Scribunto@master] Add namespace description for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1080256

Change #1080340 had a related patch set uploaded (by Sbisson; author: Sbisson):

[mediawiki/extensions/Wikistories@master] Documentation messages for Story/Story_Talk namespaces

https://gerrit.wikimedia.org/r/1080340

Change #1080340 merged by jenkins-bot:

[mediawiki/extensions/Wikistories@master] Documentation messages for Story/Story_Talk namespaces

https://gerrit.wikimedia.org/r/1080340

Change #1080256 merged by jenkins-bot:

[mediawiki/extensions/Scribunto@master] Add namespace description for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1080256

Change #1082128 had a related patch set uploaded (by Msz2001; author: Msz2001):

[mediawiki/extensions/Translate@master] Add namespace description for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1082128

Change #1082128 merged by jenkins-bot:

[mediawiki/extensions/Translate@master] Add namespace description for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1082128

WMF doesn't use or maintain the Schema namespace in EventLogging anymore. The Data-Products team currently owns EventLogging, but I don't expect them to do any work here. I believe they'd like to one day deprecate EventLogging extension generally.

Change #1085582 had a related patch set uploaded (by Msz2001; author: Msz2001):

[mediawiki/extensions/CentralNotice@master] Add namespace description for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1085582

Change #1085582 merged by jenkins-bot:

[mediawiki/extensions/CentralNotice@master] Add namespace description for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1085582

Change #1114072 had a related patch set uploaded (by SomeRandomDeveloper; author: SomeRandomDeveloper):

[mediawiki/extensions/UploadWizard@master] Add namespace descriptions for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1114072

Change #1114072 merged by jenkins-bot:

[mediawiki/extensions/UploadWizard@master] Add namespace descriptions for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1114072

Change #1116813 had a related patch set uploaded (by Jforrester; author: Jforrester):

[mediawiki/extensions/CentralNotice@wmf_deploy] Merge remote-tracking branch 'origin/master' into wmf_deploy

https://gerrit.wikimedia.org/r/1116813

Change #1116813 abandoned by Jforrester:

[mediawiki/extensions/CentralNotice@wmf_deploy] Merge remote-tracking branch 'origin/master' into wmf_deploy for the first time since August

Reason:

Done in I2780f41fb3ab98a61f89fc27977686f3e659e6d1 instead.

https://gerrit.wikimedia.org/r/1116813

Change #1162233 had a related patch set uploaded (by Phuedx; author: Phuedx):

[mediawiki/extensions/EventLogging@master] i18n: Add namespace description for Special:NamespaceInfo

https://gerrit.wikimedia.org/r/1162233

In T373070#10256174, @Ottomata wrote:

I believe they'd like to one day deprecate EventLogging extension generally.

I strongly agree with the vision but, in practice, this will take some time. The patch above adds descriptions for the Schema and Schema_talk namespaces. The description for the former includes a note that Legacy EventLogging has been decommissioned and links to T238230: Decommission EventLogging backend components by migrating to MEP.