Pending:
https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/597913/
Move defineTag, undefineTag, deleteTagEverywhere, and purgeTagCacheAll to new ChangeTagStore service
https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/598222/
Move getSoftwareTags, listSoftwareActivatedTags, listDefinedTags, listExplicitlyDefinedTags, and listSoftwareDefinedTags to new ChangeTagLookup service
https://gerrit.wikimedia.org/r/#/c/mediawiki/core/+/598226/
Move modifyDisplayQuery and makeTagSummarySubquery to new ChangeTagQueryUtils service
Remaining work to split up the class into narrow services tracked at P11294