With some I tried to get "feedback" on why my application was unsuccessful but most wouldn't enter into any discussion. The few that did gave vague responses about other applicants being "a better fit" (even though they'd never met me), better qualified/more experience (which could be true, but I'm not convinced always was with certain more specialised roles which I had the relevant qualifications/experience for)
The frustrating irony is that I interview well, I've never reached interview stage and not been offered the job and as an army brat and then army wife I've had a lot of interviews and jobs! I've even had employers who've offered me the job but I've not been keen offer more pay etc to try and change my mind, but experience taught me quite early on "if it seems too good to be true it probably is" as I took a couple of jobs I had misgivings about and it was an absolute disaster every time, so I resolved to trust my instincts in future. Interviews aren't just the employer interviewing you, it's the potential employee deciding whether the job/employer is a good fit for them. Quite honestly if my line manager had been involved in the interview process at my last job no way I'd have taken it, it was a horrific situation that I learned too late there was a very high turnover in my role due to her and that the company knew what she was like. What it does mean is that if I am ever in a situation like that again I will handle it very differently.
The 2nd boss I had after DX was not at all understanding. Indeed what actually happened in that role was my mh DX was used against me (quite literally - gaslighting, deliberately triggering me, disclosing to other employees unnecessarily etc) leading to me hitting another period of severe illness from which I am still really struggling to recover.
I was very lucky that the first employer I had after my first breakdown was very understanding. Although she didn't know when I was offered the job I disclosed to get shortly after as I needed to attend some appointments and she was incredibly supportive, unfortunately that job was in a place that was a franchise chain and the chain went bust. I was never off sick in that job even though against advice I returned to full time work less than 6 months after a full on crisis. I'm sure this was because the boss and my colleagues were professional, kind and thoughtful people who even though they'd no prior experience with anyone with a mental illness, let alone as severe as I can be, were supportive but basically didn't treat me like I had 2 Fucking heads which I think some respondents on this thread would do.
Aside from in that last job I've worked since I was 14, full time from age 16 excepting when I was at uni or on mat leave and was very rarely off sick, certainly no more than anybody else and not the type to take to take a sick day for "sniffles", a "grafter" at work happy and willing to go above and beyond when required, this is why I have good refs.