The default message (centralauth-login-error-locked) comes from CentralAuth. However in order to implement the suggested customization we'll need to create a Wikimedia override in WikimediaMessages indeed. However I'll probably change the wording of the message and point to [[m:Special:Contact/stewards]] instead given that we don't want locked users to appeal with IP addresses on Meta and elsewhere.

Change 491242 had a related patch set uploaded (by MarcoAurelio; owner: MarcoAurelio):
[mediawiki/extensions/WikimediaMessages@master] Create a Wikimedia override for 'centralauth-login-error-locked'

https://gerrit.wikimedia.org/r/491242

I kind of feel like it would be nice to include the log message from the account lock in this message, but that's kind of complex to do.

In T216347#4961392, @Bawolff wrote:

I kind of feel like it would be nice to include the log message from the account lock in this message, but that's kind of complex to do.

Yes, could be nice. If the message supported a parameter $1 for the username, we could use it to put a link to the global account log (and other functions like GRAMMAR/GENDER). I don't know how to do that unfortunatelly and it probably deserves its own task but the message seems to be defined at https://codesearch.wmflabs.org/extensions/?q=centralauth-login-error-locked&i=nope&files=%5C.php&repos=Extension:CentralAuth.

Change 491242 merged by jenkins-bot:
[mediawiki/extensions/WikimediaMessages@master] Create a Wikimedia override for 'centralauth-login-error-locked'

https://gerrit.wikimedia.org/r/491242

Will be rolled during the next MediaWiki trains, depending on the wmf branch this patch is included to. Expect one or two weeks (a bot will tell us shortly when).

It looks we'll start to have rolled it on wmf wikis starting from tomorrow (so by Thursday the message should be there for all Wikimedia wikis).