The summary says "in core", but I note there is still no answer to https://www.mediawiki.org/wiki/Talk:Typography_refresh#Wikipedia_or_MediaWiki.3F. The FAQ says the change was designed for Wikimedia sites, hints it may actually happen for everyone and doesn't explain how non-Wikimedia users' needs were considered. I also don't see a notification to mediawiki-l. Until clarified I can only assume this will be a Wikimedia-only change, presumably via configuration setting(s).
Nemo