Sure, but without remembering what repos existed when or had how many stars when, that'll be easy to forget or be hard to verify. Rather than a stupid profile badge, I'd rather see like, a repo badge, maybe linking to the latest commit that made it into the code vault.

Not all public repositories, all active public repositories. The homepage[0] clarifies that this means "every repo with any commits between the announcement at GitHub Universe on November 13th and 02/02/2020, every repo with at least 1 star and any commits from the year before the snapshot (02/03/2019 - 02/02/2020), and every repo with at least 250 stars"

[0] https://archiveprogram.github.com/

They only encoded 21TB worth.

Also, you can see which repos of yours were selected with the "Arctic Code Contributor" badge in the highlights section of your profile.

In my case they only grabbed one library that I was half-finished on... I'm pretty sure they grabbed everything they thought would be important and filled remaining space with random selections

Well my dotfile repo was archived, which to my knowledge has never had any viewers other than me. I think it may well be all of GitHub, or at least all repos that pass some measure of uniqueness