This policy throws errors to the console and addresses an old issue in Safari that has since been fixed. The issue was introduced somewhere in 2017 and fixed with Safari 11.1 in march 2018. Removing this workaround drops the referrer policy of those older browser versions into our next policy of 'origin'.
We introduced this in 2015 (T154702) documented at https://wikitech.wikimedia.org/wiki/Analytics/Data_Lake/Traffic/Pageview_hourly#Changes_and_known_problems_since_2015-06-16
The usage was last evaluated in 3 years ago in 2020 in ticket T248526: Review referer configuration of origin/origin-when-crossorigin/origin-when-cross-origin
According to https://analytics.wikimedia.org/dashboards/browsers/#all-sites-by-browser/browser-family-and-major-tabular-view Safari below 13 is now not even registering over 0.1% of users.
Safari 11.3 is also the lowest version of Safari to receive Javascript from MediaWiki since earlier this year.
I think the value of having this extra category is not worth it any longer especially as it throws an error in the console that people sometimes ask questions about.