Which version of it? 1.0?
.NET 4.6 is the version that shipped in Win10 back in 2015. Win11 ships with .NET 4.8. Thus, both OSes can run any apps written for .NET 4.x out of the box. I would expect this to remain true for many years to come, given that Windows still supports VB6.
.NET 3.5 runtime (which supports apps written for .NET 2.0 and 3.0, as well) is also available in Win10 and Win11, but it's an optional feature that must be explicitly enabled - although the OS will automatically prompt you to do so if you try to run a .NET app that needs it.
ASP.NET Web Forms sites are especially stuck, and on top of that I have customers that have developed Web Forms sites in VB.NET! Luckily there are some good bulk-conversion tools available now, but still, there is no smooth upgrade path for a lot of popular systems.
Similarly, Windows Communication Foundation, Workflow Foundation, and a bunch of other popular libraries or frameworks are dead in the water. SAP Crystal Reports is surprisingly common, but doesn't even have an official NuGet package!
But if you have developed a .NET Framework app that you want to see living into the future a decade or more from today, you need to know that maintaining it is going to be increasingly painful as time goes on. You are locked to an old version of the C# language, and you absolutely can't count on third-party dependencies to stay supported. At some point MS might well decide to drop tooling for .NET Framework development in new releases of Visual Studio.