Bastion is a simple yet effective way to build a "web of trust" for Discord servers.
Bastion keeps track of who invited who by storing a list of "invited by" relationships. This is done by disabling Discord's default invite link functionality and replacing it with a custom invite link that Bastion generates. Bastion then stores the relationship between the inviter and invitee.
If you run servers that are high trust and require vouching for new members, Bastion can help you keep track of who invited who.
Bastion, by design, is self-hosted only. It's easy and free to run using Cloudflare Workers.