This gets mentioned pretty much every time HyperCard is --- but I can't see that anyone has done anything with it.

Why use it rather than Livecode (aside from the licensing of the latter) or Hypernext Studio?

Some programs, games, and zines made with Decker: https://itch.io/games/tag-decker

Unlike LiveCode (or so far as I am aware HyperNext), Decker is free and open-source: https://github.com/JohnEarnest/Decker

HyperNext doesn't appear to be actively developed; the most recent updates I see are from last year, and it can't be used on modern computers. Decker's most recent release was yesterday morning.

I'd be happy to go into more detail if you like.

Livecode used to be opensource, which made me want to use it, but that window closed.

I guess I want a Flash replacement....

https://ruffle.rs/ recently came to my attention when I needed to resuscitate a back into tool that had been completely built in Macromedia products

There's a fair amount of usage of it on Itch.io, if you are into that indie crowd. I was skeptical of it at first -- the whole 1-bit dithering aesthetic seems a bit too retro-twee, but I find it it is the best Hypercard-alike in terms of functionality -- it "just works" as compared to most Hyperclones that seem more like a proof of concept than a functional program.