As per T165149, the contract of `Title::getTalkPage` changes to throw an exception if called on a page in a namespace that does not have a talk namespace associated.
All calls to `Title::getTalkPage` must either be changed to `getTalkPageIfDefined`, or they must first check `canHaveTalkPage()`. These checks can be omitted only for the User namespace, since user pages can always have talk pages. Note that some of the below may be calls to `User::getTalkPage` instead; those also don't need checks, since users can always have talk pages.
Offending calls to getTalkPage() should be removed from all extensions. They MUST be removed from all extensions deployed on the Wikimedia cluster before merging Icee208dc.
Here is a list of files with potentially offending calls to getTalkNamespace():
[] https://github.com/wikimedia/mediawiki-extensions-EducationProgram/blob/master/includes/UserRolesMessage.php
[] https://github.com/wikimedia/mediawiki-extensions-WikiLove/blob/master/WikiLove.hooks.php
[] https://github.com/wikimedia/mediawiki-extensions-BlueSpiceFoundation/blob/master/includes/utility/ArticleHelper.class.php
[] https://github.com/wikimedia/mediawiki-extensions-BlueSpiceExtensions/blob/master/ContextMenu/includes/api/BSApiContextMenuTasks.php
[] https://github.com/wikimedia/mediawiki-extensions-LiquidThreads/blob/master/pages/ThreadPermalinkView.php
[] https://github.com/wikimedia/mediawiki-extensions-Echo/blob/master/includes/api/ApiEchoUnreadNotificationPages.php
[x] ~~https://github.com/wikimedia/mediawiki-extensions-Echo/blob/master/includes/formatters/MentionPresentationModel.php~~ (User objects only)
[] https://github.com/wikimedia/mediawiki-extensions-Echo/blob/master/includes/formatters/RevertedPresentationModel.php
[] https://github.com/wikimedia/mediawiki-extensions-Echo/blob/master/maintenance/generateSampleNotifications.php
[] https://github.com/wikimedia/mediawiki-extensions-News/blob/master/NewsRenderer.php
[] https://github.com/wikimedia/mediawiki-extensions-InterwikiIntegration/blob/master/InterwikiIntegration.hooks.php
[] https://github.com/wikimedia/mediawiki-extensions-Favorites/blob/master/FavoritelistEditor.php
[] https://github.com/wikimedia/mediawiki-extensions-GoogleNewsSitemap/blob/master/FeedSMItem.php
[] https://github.com/wikimedia/mediawiki-extensions-Favorites/blob/master/SpecialFavoritelist.php
[] https://github.com/wikimedia/mediawiki-extensions-SiteScout/blob/master/includes/SiteScoutXML.class.php
[] https://github.com/wikimedia/mediawiki-extensions-SiteScout/blob/master/includes/SiteScoutHTML.class.php
[] https://github.com/wikimedia/mediawiki-extensions-ConventionExtension/blob/master/api/ApiConferencePageEdit.php
[] https://github.com/wikimedia/mediawiki-extensions-ConventionExtension/blob/master/api/ApiAuthorSubmissionEdit.php
[] https://github.com/wikimedia/mediawiki-extensions-ConventionExtension/blob/master/api/ApiConferenceLocationEdit.php
[] https://github.com/wikimedia/mediawiki-extensions-CommentPages/blob/master/CommentPages.php
[] https://github.com/wikimedia/mediawiki-extensions-Duplicator/blob/master/Duplicator.page.php
[] https://github.com/wikimedia/mediawiki-extensions-EditSubpages/blob/master/ExtEditSubpages.class.php
[] https://github.com/wikimedia/mediawiki-extensions-WatchSubpages/blob/master/WatchSubpages_body.php
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[] https://github.com/wikimedia/mediawiki-skins-Tempo/blob/0ca6b735b631fe97332ae0ca1768bf6e2fcf8e61/templates/NotificationsMenu.tmpl.php
More can be found using <https://github.com/search?l=&p=7&q=org%3Awikimedia+getTalkPage+language%3APHP&ref=advsearch&type=Code&utf8=%E2%9C%93>