Huh, fortunate for it that GitHub (especially) preserves it then!

I wonder how guaranteed that is, can imagine GH deciding to 'clean' it (how much data can you put there theoretically? Or to close a range of potential security issues) and suddenly relatively niche but genuine uses like jj's change-id are unintentionally not preserved.

Cleaning it would change the commit ID, so a forge cannot clean it even if it wanted to (not without rewriting all descendant commits too and breaking signed commits, at least).

The change-id is exactly as much part of the commit object as the author/committer name/timestamp, description, parent commit ID, tree, and participates in the commit hashing as those do.