This hook seems to have been introduced in the Parsoid service.
Using git blame:
- Introduced as ParsoidFetchTemplateData in https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/+/494817
- Renamed to ParserFetchTemplatData in https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/+/552666/3/extension/src/Config/DataAccess.php
So while right now it is not technically a MediaWiki core hook, Parsoid is being bundled with MediaWiki in 1.35.0, and meant to be eventually integrated into core. Currently Parsoid is able to be registered as a MW extension AFAIK but should not be considered an actual extension, per the commit message in https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/+/514356.
In the mean time, the source
template parameter (for Template:MediaWikiHook) won't work, since the hook is in Parsoid's source code, not core. I'm not sure if it would be relevant to update {{MediaWikiHook}}, since this seems to be a very unique and special exception, thoughts?
I'll document all of this information (related to it being belonging to Parsoid currently) on the page itself.