Note this is k7 - dev has since moved to k9. k7 will be remembered as the 'Unicode symbols' experiment of k; for example nulls were slashed zeroes (Ø) and pi was, well, the pi symbol. k9 dropped them as they're not easy to type across all OSes without config or a dedicated editor.
k9 also has the nice dedicated 'cut' symbol, and a few other improvements, but it's not done yet.
Please do correct me if I'm wrong, but I've just recently researched APL and it seems like beyond J there really isn't anything else. There's a lot of recent research however, so maybe soon you'll see something new, but in general array based languages seem very little used and studied.
proprietary: k7, k2/k3 (very frequently used so including them separately), dyalog APL, APL2 (still shipping), APL2000, D
not proprietary: NARS, ngn/k, J, klong, aplette, gnu apl, kona
javascript: ngn/apl, oK
calculator (free software): ivy
dead but the body's still warm (free software): xxl, nial,
dead but the body's still warm (proprietary): aplx, a+
There's a bunch missing here because everyone alive has written an APL interpreter at some point (Ken Thompson did for example but it's ancient history and has a few descendents), but listing all of them would take hours. Especially if we're getting into historical stuff.
They've been pretty heavily studied, though, certainly:
I don't really see it as still alive: half of the commits in that repo in total were on that day, and only one was a change of code. Before it, the last code change was two years prior.
Correction now that I'm not on my phone: A+ was GPLed in jan 2001, making this 19 years ago - and indeed, the last change was in 2008 for 64-bit support, but at that point that project had been already quite stagnant for 5-6 years -- changes and releases were all concerned with documentation and newer build environments.
The "electric" GUI concept (The "+" in "A+" - without the GUI it was just called "A") was much simplified in k2 - it was fast, simple and extremely effective even though it didn't really look good - it is, in fact, the only environment I've ever had a chance to use where having a GUI is as simple or simpler than writing a batch program -- see e.g. 15-Puzzle[0] or the spring simulation[1] (even though the canonical example is the S- spreadsheet which you can find at the same site).
While I can't speak for people who downmodded it initially, it was probably downmodded because "sarcastic acronym guessing game" is a trope on this site that people see frequently and are getting tired of: it seems like it at first glance, especially given that there were already people implying it was a programming language by the time you sent your comment. However, APL is niche, so you probably were well-intentioned rather than playing a part in that.
It's hard on Internet forums to see if something is genuine or sarcastic, so sometimes even the nicest comments get downmodded.
That said, complaining about them only invites more. You have a higher chance of getting back to a positive score by not bringing it up, and it's against the News Guidelines to, anyway:
Please don't comment about the voting on comments. It never does any good, and it makes boring reading.
Yeah, I was sincere. I thought it was a Public License of some sort. Never would have guessed that "APL" was "A Programming Language" without searching.
Now I know a little more about what is acceptable, I guess.
The programming language is the first thing I think of when I see "APL", and the others are unknown to me, but I still would have upvoted if phaedryx had just posted "APL = https://en.wikipedia.org/wiki/APL_(programming_language)" or similar, and put the rest of the post below that. Starting out with the wrong hits is very distracting. The self-deprecation at the end is also totally unnecessary.
https://kparc.io/kc