> You need to know how git works and GitHub's API.
No; just knowing how git works is enough to understand that force-pushing squashed commits or removing branches on remote will not necessarily remove the actual data on remote.
GitHub API (or just using the web UI) only makes these features more obvious. For example, you can find and check commit referenced in MR comments even if it was force-pushed away.
> was deeply surprised about GitHub's brute-forceable short commit IDs
Short commit IDs are not GitHub feature, they are git feature.
> If I use GitHub's web interface to click myself a repository with complex access rights, I assume they have concepts in place to thoroughly enforce these access rights.
Have you ever tried to make private GitHub repository public? There is a clear warning that code, logs and activity history will become public. Maybe they should include additional clause about forks there.