Archiving the repositories will make them read-only, which I think is appropriate since as far as I know, there are no plans to add anything more to these repositories.
Edit: based on feedback here are two more repositories to archive:
Makes sense. Going through it a bit more thoroughly, it looks like the llvm branch has all the local changes that were applied. I looked at main and it just said it was 18 commits behind, so made the poor assumption that there must not be anything.
Archiving no longer used repositories sounds good to me. GitHub allows unarchiving repositories, so this can always be easily undone, on the off chance that it turns out to be necessary.