Are user images garbage-collected at some point, or do they stay forever?

[yajo]

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We're considering using Github for functional documentation.

The markdown editor is very helpful because functional users can drag and drop images, but those get uploaded under user-images.githubusercontent.com, not in the repo.

Will these images stay forever, just like if they existed in Git, or are they garbage-collected under some circumstance such as a user deleting their account?