Currently there is around 90 files directly under includes/ and range from TemplateParser to DummyLinker to BadFileLookup. This doesn't make a great developer experience and can be overwhelming to people who are new to mediawiki.
See the parent ticket for the suggested moves.
[x] includes/ActorMigration.php
[x] includes/ActorMigrationBase.php
[ ] includes/AutoLoader.php
[x] includes/BadFileLookup.php
[ ] includes/BootstrapHelperFunctions.php
[x] includes/CategoriesRdf.php
[x] includes/Category.php
[x] includes/CategoryViewer.php
[x] includes/CommentStore.php
[x] includes/CommentStoreComment.php
[ ] includes/ConfiguredReadOnlyMode.php
[x] includes/ContentSecurityPolicy.php
[ ] includes/DefaultSettings.php
[ ] includes/Defines.php
[x] includes/DeprecatedGlobal.php
[x] includes/DerivativeRequest.php
[ ] includes/DevelopmentSettings.php
[x] includes/DummyLinker.php
[x] includes/EditPage.php
[ ] includes/Emptiable.php
[ ] includes/EventRelayerGroup.php
[ ] includes/ExtensionInfo.php
[x] includes/FauxRequest.php
[x] includes/FauxRequestUpload.php
[x] includes/FauxResponse.php
[x] includes/FileDeleteForm.php
[x] includes/ForeignResourceManager.php
[x] includes/FormOptions.php
[ ] includes/GitInfo.php
[ ] includes/GlobalFunctions.php
[x] includes/HeaderCallback.php
[ ] includes/Hooks.php
[x] includes/Html.php
[x] includes/HtmlHelper.php
[ ] includes/LinkFilter.php
[x] includes/Linker.php
[x] includes/ListToggle.php
[ ] includes/MWTimestamp.php
[x] includes/MagicWord.php
[x] includes/MagicWordArray.php
[x] includes/MagicWordFactory.php
[ ] includes/MainConfigNames.php
[ ] includes/MainConfigSchema.php
[ ] includes/MediaWiki.php
[ ] includes/MediaWikiServices.php
[ ] includes/MediaWikiVersionFetcher.php
[x] includes/MergeHistory.php
[x] includes/MovePage.php
[ ] includes/NoLocalSettings.php
[ ] includes/OutputHandler.php
[ ] includes/OutputPage.php
[ ] includes/PHPVersionCheck.php
[x] includes/PageProps.php
[x] includes/PathRouter.php
[ ] includes/Pingback.php
[ ] includes/PlatformSettings.php
[x] includes/ProtectionForm.php
[ ] includes/ProxyLookup.php
[x] includes/RawMessage.php
[ ] includes/ReadOnlyMode.php
[ ] includes/ServiceWiring.php
[ ] includes/Setup.php
[ ] includes/SetupDynamicConfig.php
[ ] includes/SiteConfiguration.php
[ ] includes/SiteStats.php
[ ] includes/SiteStatsInit.php
[ ] includes/Status.php
[ ] includes/StreamFile.php
[x] includes/StubGlobalUser.php
[x] includes/StubObject.php
[x] includes/StubUserLang.php
[x] includes/TemplateParser.php
[x] includes/TemplatesOnThisPageFormatter.php
[x] includes/Title.php
[x] includes/TitleArray.php
[x] includes/TitleArrayFromResult.php
[x] includes/TitleFactory.php
[x] includes/TrackingCategories.php
[ ] includes/WebRequest.php
[x] includes/WebRequestUpload.php
[x] includes/WebResponse.php
[ ] includes/WebStart.php
[x] includes/WikiMap.php
[x] includes/WikiReference.php
[x] includes/config-merge-strategies.php
[ ] includes/config-schema.php
[x] includes/config-vars.php