In the list=tag API query module, the tag source type named "extension" is being renamed to "software" [1]. As of MediaWiki 1.42, "extension" still appears alongside "software" in the tag source lists but is deprecated [2]. In future versions of MediaWiki, the "extension" entries will no longer appear.
The use of "extension" is misleading since it does not exclusively refer to tags defined by MediaWiki extensions (via the onListDefinedTags hook), but also those defined in MediaWiki core (via ChangeTagsStore::DEFINED_SOFTWARE_TAGS). The distinction isn't really useful to clients anyway.
(this message was cross-posted from wikitech-l)