You can use dependencies that aren't using nullable reference types in projects that use it. You can enable/disable nullable reference types per file, as it only influences static analysis. There's no runtime difference between a non-nullable reference type and a nullable reference type.
It is relatively easy to find corporate libraries, or commercial products that still aren't using it, including Microsoft products still stuck in .NET Framework, or .NET standard 2.0.
If you want name shaming of commercial products with modern .NET, here is one, can provide more.
https://github.com/episerver/content-templates