Hard-deprecated methods can be removed when they have been emitting deprecation warnings by calling wfDeprecated for at least three months, and the call to wfDeprecated is included in at least one stable release (see the [[https://www.mediawiki.org/wiki/Stable_interface_policy|Stable Interface Policy]]).
As of April 2024, all methods that had been hard-deprecated in the 1.41 release can be removed (See https://phabricator.wikimedia.org/source/mediawiki/browse/REL1_41/ for the code of that release).
Code search can be used to find calls to wfDeprecated. For instance, to find methods that have been soft-deprecated in 1.39 or earlier, use https://codesearch.wmcloud.org/search/?q=wfDeprecated%5C%28.*%2C+%271%5C.3&files=&excludeFiles=&repos=MediaWiki+core
Note however that the version number in the call indicates when the code was //soft// deprecated by tagging it. Sometimes, a method was soft-deprecated for many released but only got hard deprecated recetnly - so a method that says 1.35 in the wfDerpecated call can //probably// be removed, but if that call to wfDerpecated wasn't present in the 1.41 release, then it can't be removed yet.
Also, codesearch should be used to double-check that the method in question is no longer used in any deployed code: <https://codesearch.wmcloud.org/deployed/>. If remaining (ungated) usages are found, a ticket should be filed abotu that issue.
NOTE: Each method should be removed in a spearate patch. That makes it easier to revert the change in case problems arise.