[−3.7]: https://sharplab.io/#v2:EYLgHgbALAPgAgZgARwExIMJIN4FgBQSRKyc...
One approach to adding nullable annotations to a large code-base is to go "bottom-up" on DLL at a time. The annotations are somewhat viral; if you were to go top-down then as you get to more of the "shared" DLLs and add annotations you'll find yourself having to go back and revisit the DLLs you already migrated. In this light, the .NET standard library situation is problematic.
Imagine implementing IEnumerable<T> by wrapping another IEnumerable<T>; in .NET framework there are no nullable annotations, so you can get into a situation where you don't add a nullable annotation where you will eventually need one upgrading to newer .NET. This can bubble up throughout your code base, making that eventual .NET upgrade more challenging.
I'm not saying its not worth it to do this in .NET framework, but you can very easily add extra work to the inevitable(?) update to .NET 8+. When you get there you could of course temporarily regress and turn nullable stuff back to warnings until you work through that surprise backlog, but nullable is really nice so you might be strongly inclined to not do that... not a huge problem, just something to be aware of!
This shouldn't be a problem when multi-targeting, but we've opted to not use that for now for other reasons (performance in Rider, mostly).
It is made worse, by being a C# 8 feature and not usable in libraries that need to stay compatible with .NET Framework (yep still plenty of stuff going on there).