The Nintendo Switch Online app for smart devices offers various features, such as SplatNet for Splatoon 2 or NookLink for Animal Crossing: New Horizons, but also voice chat for compatible games, and more. There’s also a much less well-known feature to be found: the Online Lounge.
Online Lounges allow you to invite friends for online play. You send invites via URL on social networks, and by clicking it, your friends arrive at your Online Lounge, from where you can organise your play session.
If you’ve never heard of that feature, it’s not too surprising (unless you’re a diehard Splatoon fan): it was only ever used in Splatoon 2. And from July 28th (North America) / July 29th (Europe, Japan), it will not be used anywhere. It will be discontinued on that date, presumably because not enough people were using it.
Of course, other online features for Splatoon 2 will not be impacted, just like the other Nintendo Switch Online features. You will still be able to use voice chats with friends in Splatoon 2 (and other compatible games).