Timeline:
* [x] Deprecate in MW 1.33
* [ ] Remove uses around the codebase:
[x] MediaWiki
[x] `mediawiki.htmlform.checker`
[x] `mediawiki.page.gallery` https://gerrit.wikimedia.org/r/684059
[x] `mediawiki.special.userlogin.signup.js`
[x] 3D – https://gerrit.wikimedia.org/r/773381
[x] CentralNotice – `ext.centralNotice.adminUi.campaignManager` https://gerrit.wikimedia.org/r/c/mediawiki/extensions/CentralNotice/+/831137
[x] ExternalGuidance – https://gerrit.wikimedia.org/r/c/mediawiki/extensions/ExternalGuidance/+/795632
[x] GettingStarted – https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GettingStarted/+/831135/
[x] Flow – `ext.flow.components` and `ext.flow` https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Flow/+/831138/
[x] Graph – `ext.graph.sandbox`
[x] InputBox – `ext.inputBox` https://gerrit.wikimedia.org/r/683034
[x] Kartographer – `ext.kartographer.box` and `ext.kartographer.visualEditor` https://gerrit.wikimedia.org/r/829286
[x] LiveChat – https://gerrit.wikimedia.org/r/c/mediawiki/extensions/LiveChat/+/831139
[x] MassMessage – `ext.MassMessage.special.js`
[x] MediaViewer – `mmv` https://gerrit.wikimedia.org/r/683039
[x] MobileFrontend – `mobile.startup` (T236427 Dev: MobileFrontend should not use deprecated jquery.throttle-debounce)
[x] RelatedArticles – `ext.relatedArticles.readMore.bootstrap` https://gerrit.wikimedia.org/r/#/c/mediawiki/extensions/RelatedArticles/+/545919
[x] Translate – `ext.translate.messagetable` https://gerrit.wikimedia.org/r/682998
[x] Wikibase – `jquery.wikibase.entityselector`, `wikibase.view.ViewFactory`, and `ext.WikidataPageBanner.positionBanner`
[x] WikiEditor – `ext.wikiEditor`
[x] (skin) Vector – `skins.vector.js` https://gerrit.wikimedia.org/r/535914
[x] (skin) MetroLook – `skins.metrolook.js`
* [ ] Remove in ? MW 1.40 – https://gerrit.wikimedia.org/r/c/mediawiki/core/+/831141