It looks like Sitecore has returned public visibility to https://github.com/SitecoreSupport
This is a GitHub repo that Sitecore’s official support team uses to document, track, and communicate the various patches they make available to customers. Public access was removed about a year ago, and some of us in the community really felt the loss (and wished we had downloaded the entire repo to use as a reference!).
For me, that repo is a wealth of information on how to tackle specific Sitecore challenges. It’s instructive to see the strategies involved with addressing a Solr integration defect, just for example, and can be applicable to a lot of different customer solutions.
A lot of my time the last few months has been focused on Solr integrations and connecting different Sitecore systems together, so having examples like https://github.com/sitecoresupport/Sitecore.Support.198901/releases or https://github.com/SitecoreSupport/Sitecore.Support.449298 can be like a master course in how the internals of Sitecore fit together.
I’m not suggesting you take the code on that repo and blindly apply it to your Sitecore implementation, but as a learning reference and source of how Sitecore’s internals behave, it’s a vital resource to those advanced Sitecore professionals.