At my very first professional EE position, in 1978, there was an older staff member, PhD, kind of a professor type, who was programming in APL. Naturally curious, I asked him about it and what the strange symbols on the keyboard meant. His explanation mostly flew right over my head but I asked a few questions, thanked him, and went away. In 43 years, he's the only person I ever saw using it.
I've always been curious about APL. My grandfather partnered with an APL programmer in the early 70's to develop shallow geodesic dome geometries suitable for home construction and optimized for least number of distinct strut lengths. The one he lived in is still standing[0]
I used it when I was in college for my Senior Project. That would have been 1978/1979. I had a keyboard with the APL symbols molded onto the keypad.
There are a handful of modern keysets available with APL legends as well
https://novelkeys.com/products/cherry-olivia?variant=4300136...
https://21kb.com/products/21kb-apl-classic-retro-beige-keyca...
https://omnitype.com/products/gmk-dimensional-cyl?srsltid=Af...
Im running APL only stickers on my keyboard because it seemed more entertaining than blank caps for touch typing. Freaks people out, but really enjoy it.