I was a co-founder in a startup. It was split 50/50. I did all the development work and my partner did the business development work.
I had to drop out due to health reasons. I retained 1%, which I felt was fair. The main thing is they needed to bring in new people to replace me and needed equity to offer them. We did formalize it, and I have the legal documentation showing my shares in the company.
They have gotten investment so I'm sure my share is diluted quite a bit by now.
That was about 5 years ago that I left. At this point I'm ok if I don't get anything out of it.