With no insider knowledge whatsoever I can assure you that they do. Large enterprise shops always have these things cached because why take a hard deploy-time external dependency when you can spin up a rubygems (etc) cache backed entirely by your object storage of choice.
Former Shopifolk here. As of 2022 - 2023 they had some private packages in their SCM and an internal Artifactory deployment that was a caching proxy of Rubygems and other upstream dependencies. This may be changed since, but as far as I’m aware many Shopify devs occasionally volunteer time and fixes to Rubygems and related projects from time to time.
They certainly have the capacity to run their own full on mirror service, but I doubt they have serious incentive to do so given exciting controls and culture re: Ruby and OSS contributions.
