Speaking as somebody who has worked as a professional Games Artist, you won't see these online slot machine art roles advertised unless you're looking for Games Artist roles. In fact, MOST of the games artist roles here in the UK are actually for companies like King who specialise in these online casino games, and it SUCKS because I'm morally opposed to doing work for the gambling industry.
I did apply for one once years back because I was desperate. They really took the piss with a ridiculously complex and involved art test that involved both UI AND Illustration asset creation, then I didn't even get the job (I was kinda relieved because I really don't want to work for a gambling company ever but at the time I really needed money... luckily I got a big freelance gig shortly after).
Here's one of the pieces I did for that art test:
So yeah, it's actually a pretty well-known job for games artists, but not one people are necessarily proud of, partially because games artists mostly hate "getting stuck on UI", but also because many would really rather be making like... fun RPGs and Adventure games than milking vulnerable people for money like that, and like all games art jobs, it's INCREDIBLY competitive.