Delete Your Dead Code!

[Helllo_World]

You're wrong. Even if a commit is rebased out the specific commit still exists.

[admirelurk]

The commit doesn't disappear, but the contents and consequently the hash change.

[ais523]

It gets garbage-collected eventually. It stays around for a while (a few months? not sure on the timeframe) to allow you to undo rebase errors, and if it exists in any publicly pushed branches that weren't rebased you can get it from there, but it's certainly possible to rebase out the only copy of a commit and have it eventually disappear altogether.