On real part-time: I think once it moves below 3 days a week it works better for transactional roles. E.g. I know dentists that work for a few days per week in a practice, people with 8-16h per week in chat-support, 3-day/wk secretaries, etc. Most of these roles measure their work in calls/appointments/hours rather than longer running projects though.

But with the right culture it can be done in IT as well, I think Gumroad is a good example.