Which framework you will use in 2021 to build stack overflow

The reason for ASP.NET is that it was what Atwood and Spolsky knew. It also helped that they knew SQL-Server scales well vertically. It was a good choice then and would be a good choice today.

Stack Overflow was able to grow huge because it was built on the Microsoft stack. For a long time it ran on a single database server...a big box with lots and lots of disks and RAM. Because it was a business from the beginning, they were willing to pay for sound technology. They could do so because they were adequately capitalized.

Good luck.

Just follow their path: choose the framework you know best.

There are two types of side projects. One you do because what you’re building is cool. The other because you’re interested in learning some tech.

If you’re focused just on the thing you’re building use whatever reasonably modern thing you want. I’d probably use flask, MySQL, and avoid a JavaScript framework as long as I could... but that’s just what I’d work fastest in. Do whatever you are most productive in.

Whatever framework you pick, just don't use Segoe UI.

ASP.NET Core