{If you do not work on MediaWiki extensions or skins on Gerrit you can safely ignore this email.}
If you are seeing this error in CI you will need to update your ResourceModule definitions inside skin.json or extension.json to remove any targets key in objects. If your code needs to be restricted to the Minerva skin or all mobile skins please see https://www.mediawiki.org/wiki/ResourceLoader/Migration_guide_for_extension_... for information.
This effectively marks the beginning of the end of the MobileFrontend target system which has served us well for the last decade but has become more of a nuisance as our codebases have matured meaning many new features have not been enabled on the mobile site either intentionally or accidentally. For more information around motivation at https://phabricator.wikimedia.org/T127268. A test failure will now be triggered for code trying to limit where code is loaded using this mechanism.
If you are a developer with ResourceLoader module definitions that define targets: [mobile,desktop] you can and should safely remove those.
Note, targets (for now) will continue to work in gadgets but may not in future. Please join the conversation at https://phabricator.wikimedia.org/T328610
Thanks for reading! Please feel free to reply with any questions.