Earlier quoted context omitted.
This too. I have an "Archive" folder, and before that one called "Dissertation" which was in reality like, 8 different projects.
This is a good solution. Have a archive private repo that everything I'm done with goes in. Perfect.
I do wish GitHub had a way to deactivate projects to save money. Maybe put it in a read-only state, still count it against your disk quota, but you'd have to make it active to actively work on it and update it. I feel like a lot of systems do similar things (eg. charge per user but allow deactivating users in case they go on leave or quit but may return--pause paying for that user without losing the data in case you want to reinstate.)