Totally agree. There is no backward compatibility with .NET runtime - if your application is built/linked to a given version, it won't work with any other version of .NET
That's simply not true. Newest .NET 8 does not need the assemblies you reference to target .NET 8 - as long as the TFM is any version of 'netstandardx.x', 'netcoreappx.x' or 'net5'+ it will work.
You can even make proxy-projects that target netstandard2.0 but reference .NET Framework and with certain compat shims the code will just run on .NET 8 unless it relies on some breaking changes (which have mostly to do with platform-specific behavior, there have been no breaking changes for the language itself since I think C# 1 or 2? some odd 20 years ago).
As for the runtime itself - the application can restrict itself from being run by a newer version of runtime but you can absolutely do so. The lightweight executable that just loads runtime and executes the startup assembly may complain but just try it - build a console app with 'net5.0' target and then run it with latest SDK with 'dotnet run mynet5app.dll' - it will work.
That generally only happens with .NET based programs in Windows systems. You always need some .NET v2,3,3.5,4,4.5, etc., runtime.