For anyone who is still wondering how to do this, 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

To display a notification, just add the following code:

// Necessary for MacOS (you can change it to your own identifier)
Notifications.BundleIdentifier = "com.apple.finder";
// The first argument is necessary, the last two are not. The third argument is only used on MacOS
Notifications.ShowNotification("notification-title", "message", "extra-info");

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.