I do know that this warning means that the package is not compatible with my project's version. In my case it's NET3.1, and the package requires NET Framework at least 4.6.1. I would change target of my project to NET Framework but I need interface default implementations which seem to not work in NET Framework.
But anyway my question is: If all used functions of the package seem to work fine and I ignore these warnings, then release my application, would it mean that my app is fully functional but just needs "both" .NET3.1 and NET Framework?
You can either use .NETCore runtime or .NET Framework runtime. For interop between the two there is .NET Standard which can be used by both runtimes.
I would personally by no means release an application ignoring this warning.