depends on exactly what we mean by "suicidal". around 4% of the US population contemplates suicide in a given year, though that number is double or more for younger people. only a minority of those who contemplate it make an actual plan, and again, only a minority of that subset actually follow through with an attempt.

if someone told me they had a serious plan to kill themselves on a first date, I would be more likely to try to get them help than to plan a second date. if they told me they periodically have suicidal thoughts, I would consider that a bit of an overshare, but not necessarily a dealbreaker. there are worse problems a partner could have.