I just git clone old projects into a Dropbox private folder. I can easily keep the whole repo history even if there are multiple branches, but it doesn't take up GitHub usage at all. Of course, if you had Issues or the like, none of these solutions really work.

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.)