it seems valid regardless of not having an https: in front, protocol-relative links are OK in external link syntax. this is probably due to some lack of parsing somewhere

Yep, I think so too. I'm trying to have a look at the code to see if I can figure out how to make the protorel links to work there, but if somebody else beat me in doing that I don't mind :)

Did someone actually worked on this one? Because on Firefox and Chrome I see the links working just fine now.

Links work fine for me too.

Someone just contacted us (luckily)
How can they find way to contact stewards when links are not working?

Seems like they are using mobile phone.

Another report of this, also from a mobile phone but using Vector. (and the initial task description's screenshot is non-mobile, so I guess this is unrelated to platform?) (sidenote that I've forwarded this specific report to -revi)

Change 569383 had a related patch set uploaded (by Urbanecm; owner: Urbanecm):
[mediawiki/extensions/GlobalBlocking@master] Fix wikitext links in "globalblocking-ipblocked-*" messages

https://gerrit.wikimedia.org/r/569383

Change 569383 merged by jenkins-bot:
[mediawiki/extensions/GlobalBlocking@master] Fix wikitext links in "globalblocking-ipblocked-*" messages

https://gerrit.wikimedia.org/r/569383