It’s still definitely a big deal. Note that the CIA and the NSA routinely declare ex-employees that whistleblow or leak as “mentally ill.”
It depends on the company. I worked for fairly “stolid” companies, for most of my career, and I suspect that they would treat mentally-ill people badly.
Mental illness is something that, unfortunately, I have a lot of experience with. I have severe mental illness in my family (I deal with it every day), and I spend a significant part of my life, interacting with folks at various stages of recovery from it.
I have been seeing therapists for much of my life. When I was a kid, I was diagnosed with autism, but was never told, so I spent decades, trying to “fix” myself, before finding out. Once I found out that I was “on the spectrum,” I realized that it can’t be “fixed”; only mitigated, and things started improving quickly, at that point.
That said, I think “mentally-ill” means “diagnosed and professionally-treated,” to most folks. It’s my opinion, that there’s a lot of undiagnosed/untreated mental illness out there. Just looking at the threads of interaction, on any Internet community, makes that clear. One “tell,” that I have encountered, is when someone has extremely strong opinions on psychiatry. It’s not something most folks even think about, so it’s unusual, when it’s a big deal to someone.
Mental illness also tends to get worse, as we get older, if untreated. An “eccentric” young man, may become an old hermit, flying around, keeping his piss in canopic jars.
Much of what we call “mental illness,” is actually self-developed coping mechanisms, in response to trauma, or brain-chemistry imbalance. That’s why getting medication doesn’t just “fix” us. We need to seek help in defusing the habits and rituals that were developed to help deal with the problem.