I'm late to the party here. Although I'm a gray hair, I didn't start using UNIX until the early '80s, and though I've seen, and been curious about efl, I've never used it. Fortran 4 was my first high-level language in college in the '70s.
That said, I do remember efl being on an early PC based UNIX - Microport SVR2. On later Microport UNIXen it was gone, but I can't remember whether it disappeared on SVR3 or 4.
I currently play with 4.3BSD Quasijarus system on simh, which has efl. What a fun system to play with! Maybe I'll give efl a try if I can find enough docs to grok it.
Tom