I wonder what they're being used for. The UI doesn't expose it, but the Bluesky API will tell you who redeemed your invites. Open the site, watch for a "com.atproto.server.getAccountInviteCodes" request in your browser's network inspector, look in the "usedBy" field in the response JSON, and append the DID value there onto "https://bsky.app/profile/". Any commenters in the parent chain who got scraped want to take a look?