I suspect it's more common to say, for example, "I decided to forgo treatment" rather than "I forwent treatment." But why?
I think there is a tendency to avoid using non-basic forms (also known as the other 'principal parts') of irregular verbs that are themselves uncommon in the present tense. I suspect this is so because people cannot remember the proper form or find it awkward. For example, the verb slay is used a lot (slaying, slays), but how often do you read slew?
As for your other question, people tend to cling fast to the original verbs in set phrases. "Forego the opportunity" is perhaps the most famous one for "forego", and you will find that Google yields 33 000 instances of the past tense "for(e)went the opportunity." So there is at least one usage of "for(e)went" that is alive and well.