For a pr-subscribers-xx maintainer (approver for the purpose of patch subscription, not related to authority), it seems that GitHub only presents the “pending request” information on the https://github.com/orgs/llvm/teams/pr-subscribers-* page, not on any other page (confirmed by @boomanaiden154-1)
I don’t think we can expect a maintainer to visit https://github.com/orgs/llvm/teams/pr-subscribers-* pages they manage to check new pending join requests routinely. So if they miss the email notification would like to join "LLVM"'s "pr-subscribers-llvm-binary-utilities" team, the request may stay pending forever.
The current best way is perhaps to have more maintainers for the pr-subscribers-xxx teams. I guess every maintainer will receive a would like to join "LLVM"'s "pr-subscribers-llvm-binary-utilities" team email notification (with the high volume of llvm-project emails, I think the error rate is not very low), but it’s probably better than neglecting an email notification. Unfortunately it seems that a maintainer cannot give maintainer state to another member. Perhaps only administrators can do this.