​I can not speak for anyone else.   But at the time when I was a part of the /usr/group UNIX standards** mtgs  I personally do not believe I had ever heard of the term "​solidus." Such a term maybe had been used in my first form Latin classes from the 1960s, but by the 1980s  I had long ago forgotten any/all of my Latin.  I certainly did not try to remember it as a computer professional.

In those days many of us, including me, did (and still do) refer to the asterisk as "splat" and the exclamation point as "bang"  from the sound made by them when they printed yellow oiled paper @ 10 cps from the console TTY.  But slash was what we called the character that is now next to the shift key on modern keyboards.   I do not remember ever using, much less needed to refer to, the character "back slash" until the unfortunate crap that the folks in Redmond forced on the industry.   Although interestingly enough, the vertical bar or UNIX "pipe" symbol was used and discussed freely in those days.   I find it interesting that Redmond-ism became the unshifted character, not the vertical bar by the shear force of economics of the PC.

Clem

** For those that do not know (my apologies to those that do) the 1985 /usr/group standards committee was the forerunner to IEEE P1003.  Which we published as the first "official UNIX API standard agreed by the community" (I still have a hardcopy).  But neither /usr/group nor USENIX had the political authority to bring an official standard to FIPS, ANSI, ECMA, ISO or like, while IEEE did.  So a few months before the last meeting, Jim Issak petitioned IEEE for standards status, and the last meeting of the /usr/group UNIX standards meeting was very short -- about 10 minutes.   We voted to disband and then everyone in the room officially reformed a few minutes later all signing in as IEEE P1003, later to be called POSIX.  For further historical note, I was a "founding member" of both groups and the editor of a number of early drafts (numbers 5-11 IIRC), as well as the primary author of the Tape Format and Terminal I/O sections of P1003.1.  With Keith Bostic, I would later be part of the P1003.2 and pen the original PAX compromise.  After that whole mess I was so disgusted with the politics of the effort, I stopping going to the POSIX mtgs.

PPS While I did not work for them at the time, you can blame DEC for the mess with the case/character sets in the POSIX & FIPS standards.   A number of the compromises in the standard documents were forced by VMS, 7-bit (case insensitivity) being the prime one.   While we did get in the rational section of document that it was suggested/advised that systems implementations and applications code be case insensitive and 8 bit clean so that other character sets could be supported.  However the DEC folks were firmly against anything more than 7-bit ASCII and supporting anything in that character set. My memory is that the IBM folks were silent at the time and just let the DEC guys carry the torch for 1960's 7-bit US English.