https://gerrit.wikimedia.org/r/#/c/67233/ remotes the old-style login and create account forms from core. The new forms have different messages, e.g. no more colons in English labels and there's no more use of the  combined "Log in / create account".  So I removed the old messages from MessagesEn.php, MessagesQqq.php and messages.inc.

Raimond pointed out that the

 'userlogin' message is still used by the OpenID extension.

Maybe more of these very old messages are used by other extensions.Needs investigation, partly revert recommended in the meantime.

Good point, it's also used by MobileFrontEnd (!), I'll submit a patch restoring this message and ask on the i18n mailing list for guidance.

FWIW mw.org's I18n page and Manual:System_message#Creating_new_messages don't say anything about extensions reusing core messages. I think they should caution extension writers about relying on messages in core, and instead define their own messages with a {{same}} template in Qqq. Or we need to somehow identify "reliable" or "deprecated" messages in core.

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=S Page  software engineer on E3