Lots of comments here remind me of the time GitHub was purchased by Microsoft. It would be the dead of GitHub. While in fact it got better: GitHub Actions (pretty neat CI system) happend under Microsoft. Free private repos happend under Microsoft.
Now this time it could be different. But last time wasn't that bad imho.
Microsoft made the GitHub UI significantly worse by rewriting everything in React. It's now slow and bloated. Copying text from the file viewer is a nightmare. And never ever look at how GitHub Actions work under the hood, you will wish you never became a developer.
Search is very much FUBAR. I get nagged to register even though they know I have like 17 github accounts, because who does not? They missed the ball on that one. Sometimes I like to browse without any cookies on a separate browser for window shopping reasons. Leave me alone Microsoft.
I don't think they used a framework, I remember they used a lot of "vanilla JavaScript". They're using Rails after all. Here is a thread from 2022 where they began using React for their (then) new search: https://news.ycombinator.com/item?id=33576722
Gitlab had their CI/CD a few years earlier, Github had no other option. As to which one feels more productive, that's up to personal tastes, for me Gitlab's option seems far more polished.
It did not entirely get better; some things may have improved and some things may have been made worse.
Private repositories is not a feature I use (if I want the files to be private, I will not send them to Microsoft or to someone else, unless they are the intended recipient).
I use GitHub Actions to automatically assign issues to myself,
I think they have changed the HTML in many worse ways; some functions require JavaScripts, etc. They also made mandatory 2FA, and setting it up does not work properly. (I can use the API to get around both issues, for now.)
Now this time it could be different. But last time wasn't that bad imho.