How far back with travel insurance?

Insurance questions are expected to be completely unambiguous these days so stick with the wording of the question. If it asks "have you ever....." then answer accordingly, but if it only asks "In the last 5-years....." take that into account.

I don't deal with travel insurance but typically, for life insurance, insurers only ask "have you ever" for mental health problems which caused hospitalisation, referral to a psychiatrist or were there have been suicide attempts.