They are all kind of pre-multiprocess/Rust Firefox forks. It seems Pale Moon has forked Gecko into Goanna and made it embeddable (which is neat!) and that's what K-Meleon uses too. Which I didn't know.

Is Goanna on part with web standards? Maintaining what seems basically a folk of an old Gecko must be hard.

It also kinda validates my point: using Gecko elsewhere is a PITA. You have to work hard to make it embeddable.

To answer your question, Gnome Web / Epiphany was once based to Gecko. It switched to WebKit because using Gecko was harder and harder. Konqueror optionally allowed you to use Gecko, but that stopped being possible a long time ago for the same reason. Galeon and Camino both died a long time ago.

Brave, Vivaldi & Co picked Chromium instead of Gecko. With Eich coming from Mozilla, I think Brave considered Gecko but that was deemed too hard.

Brave was originally based on a Gecko-based Electron-alike. I think it was called Muon?

No, it was based on Blink: https://github.com/brave/muon

Muon was our Electron fork (μ− is heavy e-), but Brave started on Gecko:

https://brianbondy.com/blog/174/the-road-to-brave-10