This could also be an indexer that runs locally, although I don’t know how you would find unreplicated peers for indexing. I wonder if they have considered DHT or similar for announcement.
Edit: looks like they ruled out DHT in the context of advertising repositories because they want replication to be opt-in, likely to avoid replicating objectionable material, but I still think it’s a good idea for advertising nodes more generally. The curious could then use the list of nodes for indexing.
Even though I moved some of my personal repos to Codeberg I believe there needs to be a centralised way of searching in open source code.
I've personally never discovered projects through Github.
The social connectivity and discoverability in github is a big reason many people use it. Can't say how many, but it is.