My org has about 500 repos split across 3 orgs. There’s a “main” org, an OSS org, and then splinters for people who are willing to pay for and manage their own.
For the main org, we have templates that projects typically follow where they include info in the readme for what group and individuals manage and we require that they add a tag for the specific center and encourage tags for projects.
So it’s sort of possible to search for a tag within the org to see all the projects but it’s janky and confusing.
Some groups create “housekeeping repos” that is just a repo with docs that link out and describe all their projects. They use repo instead of wiki as GitHub wikis are kind of a pain to manage (must have specific permissions and can’t just fork and send a PR). So that group uses the housekeeping repo as the link they give out to new team members, etc.
For the OSS org projects, we also have a portal repo that builds a github.io portal site that shows cards for each repo and allows searching and sorting. The OSS org doesn’t use the tagging scheme because the public doesn’t really know or care about our internal org names. We have about 175 projects in our OSS org.
Note, we also run GitLab community edition internally and actually have subgroups and stuff. But since GitLab requires internal network access GitHub use is growing since GitHub is in the cloud and doesn’t require VPN. The GitLab license costs are much higher than GitHub and not really compatible with our dev style. We have lots of non-devs and the ratio is probably 3:1 of non-dev:dev and GitLab makes us license everyone the same so we can’t pay $1000/year so a PM can update readmes and project cards.