Git does forget, it has a gc mechanism specifically for forgetting.

GitHub can't use the native git gc, and apparently doesn't have their own fork-aware and weird-cross-repo-merge-aware gc, so they might just not have built a way to track which commits are dangling.

But that's not obvious at all.

How to make git an elephant: https://donatstudios.com/yagni-git-gc

Ah right, so forking the codebase, then deleting the original repo forces git to forget all copies. Gotcha thanks for the enlightenment.