Not suggesting an app but I think what you plan to do as an effort to boycott GitHub is the most non-productive thing to do…
What if it is a project like OpenBSD, which the active repo is still CVS, but has a read-only GitHub repo for public consumption? I believe a fair amount of open source developers rely partly or even totally on donation, and the popularity of a platform has direct impact to their life support. What you are going to do is the easiest as a totally unrelated onlooker - you deem them to be immoral because they host or mirror the project on GitHub, you judged just like how people do on social media when they hear some company doing evil things, spent probably just a few minutes to write a post and created direct impact.
There are many more better things to do. You know why they are forced to host on GitHub so why don’t you set up a fund that supports developers to not host on GitHub, or as a maintainer who helps projects to mirror on GitHub alternatives and eventually swtich the active repo to the alternatives?
I have great respect to open source developers who bring us a better world. Please think again what is the difference between what you are going to do vs people who ignore all the heavylifting of open source developers did and say “add this feature in the next hour or I will stop using your software, because I think it is the most important thing to do”…
That’s exactly why I replied. I think you are an upright citizen and you and many others know full well how evil Microsoft is. But the most regrettable thing to do will be to start with a noble cause and end up doing harm to people who you should help and protect.
I am not going to drill into anything political here as it is not the right board. But please think about this - helping developers to move away from Microsoft is punishing the evil; Boycotting projects that are hosted on GitHub, in hope of dealing damage to the evil, is not actually doing it. You are punishing open source developers who are contributing to a free world, and not punishing the evil.
Just like in my original reply, there are many things you can do. Codeberg is doing it by providing a better alternative to developers; Richard Stallman started GNU project and provided a big collection of useful software and libraries to help people move away and fight proprietary tools, not just slapping people who used proprietary software; Even if you help just one more open source project to host on Codeberg, people will be able to find one more project on the Codeberg and can choose not to go to GitHub for that…