There have definitely been cases where hosting services have allowed someone to link a personal account into a corporate one belonging to their employer, then at the end of the employment the corporate account has been given control of everything within the personal account. I don't recall whether GitHub specifically was one of the services mentioned, but I would avoid creating that kind of link on any hosting service where I had my own data. Maintaining clear separation between personal and professional devices and accounts is a sound policy and there are very few sensible reasons not to follow it.