I'm doubting about this. Afaik it's still bad practice to use both? Nothing goes wrong but it's still obsolete IMO

The ILoggerBuilder interface is not obsolete. And you are not required to use IHostBuilder.

But it would be nice to have a better fluent-interface-integration with ILoggerBuilder and IHostBuilder to load the NLog-config-file depending on environment (Ex. NLog.Dev.config and NLog.Production.config)

Not everyone setup logging upfront before building the IHostBuilder (Ex want to load environment specific nlog.config). With NetCore3 you will have a lot of confused people that can no longer use ILoggerFactory. Instead a good alternative would be ILoggerBuilder.

I believe it should be possible to setup NLog as LoggingProvider directly on LoggerFactory.Create:

var loggerFactory = LoggerFactory.Create(builder =>
{
    builder.ConfigureNLog("NLog.config");
});
ILogger logger = loggerFactory.CreateLogger<Program>();
logger.LogInformation("Example log message");

And Microsoft recommends that logging is setup for IHostBuilder using IHostBuilder.ConfigureLogging:

public static IHostBuilder CreateHostBuilder(string[] args) =>
    Host.CreateDefaultBuilder(args)
        .ConfigureLogging(logging =>
        {
            logging.ClearProviders();
            logging.ConfigureNLog("NLog.config");
        })
        .ConfigureWebHostDefaults(webBuilder =>
        {
            webBuilder.UseStartup<Startup>();
        });

Ok merged! thanks!