can someone say if this extension works with MW1.30.x?
Topic on Extension talk:Arrays
It does, yes.
Unfortunately, at least version 491256a (last one from REL1_30) leads to problems with Mobile Frontend and the Minerva skin. The little arrows at the sections are not displayed, and the sections won't collapse again once they have been expanded once. I'll investigate that problem further. It's a pity that it isn't maintained, but perhaps someone has the skills to fix that bug, so if I can reproduce it I'll report it at Phabricator.
It works with the "master" branch (version 2166695). And it seems to be maintained still. *phew*
Why would this be "abandoned"? I have found it useful for my wiki. Not sure how to do some things otherwise. Are there suitable alternates if this truly is abandoned?
It's not abandoned, it simply doesn't currently have a maintainer. The current version works with MediaWiki 1.30, and was released less than a year ago, though, so even without being actively maintained I expect it to continue working for a while.