I managed to find some Objective-C code to do it with, which can be P/Invoked through C#. Since it seems no-one has made a cross-platform library for this before (which I do find quite weird), I just made a library myself which shows notifications using built-in Linux/MacOS/Windows notification managers: OsNotifications. You can just install it via NuGet:

dotnet add package OsNotifications

Currently it only shows very basic notifications, but it works on the three OSes mentioned before. I will continue developing it according to the people's needs, so if anyone sees this and is missing a specific feature, just create an issue on GitHub.