As above, seems to be maintained now. I even couldn't use master on 1.35 (requires 1.37), so should the warning be removed?
Topic on Extension talk:MaintenanceShell
In other words: Master of this extension works for you on MediaWiki 1.35? If yes, this will be cool and indeed the warning could be removed.
Master is too new for 1.35 (think it needed 1.36 or 1.37), but the version served through the extension downloader for 1.35 (which is recent) works really well.
Thanks for your feedback. I updated the page to reflect your feedback.