I believe that the Discourse team is not very interested in creating such a system. Personally, I shy away from it due to the risk of accidentally using secret knowledge (e.g. “this private/unlaunched forum exists”, etc - especially if they’re our customers) while building up the data, so we’ve left it to others who are motivated to catalog public knowledge, such as Google (who don’t currently seem interested).
Additionally, there have been bad experiences with people spamming the contact email of forums; making a canonical list would make the spammers’ job easier.