People used to use shell variables like this a lot in (to name one place) alt.sysadmin.recovery, such that the FAQ even mentions it: http://www.faqs.org/faqs/sysadmin-recovery/ (section 1.3)... meaning people have done this for decades.

> $USEE $OF $SHELL $VARIABLES,

That's obviously being done ironically and not at all similar to the commenter above's use of $OLDJOB.

I remember seeing this done all the time, back in the 90's on Usenet. It was very common.

Just search for it. An example from a 2016 SANS blog post:

https://isc.sans.edu/diary/Applied+Lessons+Learned/21107