I think discussing physical health problems is often taboo as well. Maybe you're in a place where that is more accepted.

For both physical and mental health problems, the social acceptability of discussion depends on a lot of factors: the context of the discussion (who, why, where) is definitely important, some individual conditions are more shameful than others, and a lot of people will get hostile if they feel your condition is causing them any inconvenience, especially if it's an "invisible" condition.

I'm fairly open about my mental disorders, and I haven't had too much trouble from it. Granted, there are a few things other people complain about as "stigma" that I'm perfectly okay with. (On another forum, discussing lack of ADHD acceptance, one person complained of other people having a "this person is not normal" facial expression when she told them she had ADHD. I get surprised facial expressions. It seems reasonable to me that people would be surprised and therefore have a surprised facial expression.)

Most of the discrimination/stigma/taboo I see is based on individual traits/symptoms and often applies to people who have those traits without mental illness as well.