Sounds like a fun weekend project to replicate it?

The GitHub search API seems to provide everything that is needed. It returns 30 repos per query and allows 30 * 60 * 24 = 43200 queries per day. So one could track the top 43200 * 30 = 1296000 repos.

That is enough to track all repos with more then 20 stars:

https://api.github.com/search/repositories?q=stars:%3E1000&s...

But maybe it has already been done? This looks like it:

https://ossinsight.io/#trending-repos