This tool helps query your own GitHub repos to see which licenses you've used. It would be pretty easy to adapt this to query any user or organization's public repos, but I'll leave that as an exercise for the reader.
Open the GitHub GraphQL Explorer (or any GraphQL client) and run the provided query. If pageInfo.hasNextPage
is true, use the provided cursor to query for the next page, see Using pagination in the GraphQL API for more info. Merging multiple pages for use by the JS collator script is also left as an exercise for the reader.
Then, run the JS code to collate the results, it's probably easiest to do in your browser's JS console but any JS REPL such as Node.JS will do.