Not familiar for anyone born after the 90's.
Last release was 2014. Project is still hosted on SourceForge. Website copyright notice ends in 2018. Last commit to the github repo was 2018. Project front page mentions the original Xbox, Minix, and Zaurus (and early 2000s Japanese handheld organizer that ran Linux) and Solaris as targets.
This should give you some idea of what time frame to view any claims from the website in.
I don't see that as negative, but a remark that most younger people are not used to the look of Windows up to (and including) 2000. And the last time I personally used FLTK had been around 2000 too. Which, btw, (at least back than) looked more like Irix' 4DWM controls than Windows.
I imagine Ede would play perfectly with Tiny Core Linux [1] which has FLTK/FLWM as its default desktop system.
The only system that doesn't look like either, that I can think of, is Plan 9.
Which was inspired by NextStep.
https://en.m.wikipedia.org/wiki/IBM_Common_User_Access
CUA predates NeXT by quite a lot and so the influence would mostly be going the other way.
CUA was the design paradigm chosen for OS/2, a joint project between IBM and Microsoft. When Microsoft started the Windows Project ( after OS/2 ) they pretty explicitly adopted CUA for it as well.
Meanwhile, XForms has been open-sourced, but FLTK being in a different language and having evolved a bit since its creation didn't suffer from the problems Lesstif had and is standing on its own rather well. And despite being C++, it pops up rather often when you're looking for GUIs with decent language bindings (e.g. for Lua or Rust). Probably because it's less a moving target than Qt or, heck, Gtk.
Good times.
EDE itself though looks like a dead project. I would not expect this to survive the jump to Wayland.
