Thread:Talk:User hub/if protocol relativity mandatory or merely recommended

A [//en.wikipedia.org/w/index.php?title=Wikipedia_talk:External_links&diff=585647641&oldid=585520806 difference of views] has become apparent in Wikipedia about whether URLs must be protocol-relative or it's only recommended that they be so. While I did read the [//wikitech.wikimedia.org/wiki/HTTPS#Protocol-relative_URLs Wikitech section] as mandatory, I suppose one could argue that it's only mandatory if one wishes "to make HTTP and HTTPS coexist happily", which we normally do, in which case it could be considered to be recommended and not mandatory. And I may have found a case in which [//en.wikipedia.org/wiki/Wikipedia_talk:External_links#question_on_mirrors_especially_non-MediaWiki_and_WP_templates flexibility may be better]. Is protocol relativity mandatory? Either way, the Wikitech section should be clarified, but I can't log in to do so or even to start a talk page there. Thank you.