What do you mean? I don’t think any other operating system even comes close to Windows in regards to backwards compatibility. Most software designed for Windows 98 will still run fine on Windows 11.
There are thousands lines of code in the NT kernel with patches for specific programs. There is even a line patching an incompatibility with Lego Island, lol.
Well, the website and mobile app are overwhelmingly hated (just look at the reviews on the PlayStore). I think there are two major things helping Reddit: It’s easy to grasp (Lemmy has instances, dozens of different apps, etc.) and the fact that Reddit already has a community for basically everything.