Not many people will have noticed, but in a blog about Gran Turismo 7, Sony has confirmed this.
In the blog there is an asterix behind all mentioned modes, except arcade mode. The asterix refers to a rule that requires a constant internet connection. So if your internet is down or if there is maintenance on the server, you can’t play these modes.
So it sucks for the players with a weak internet connection! This is unfortunately not new in Gran Turismo: GT Sport also requires the same. What do you think?