All other features of the app will continue.
Nintendo has announced that the Online Lounge for Splatoon 2 will no longer be supported by the Nintendo Switch Online app as of the 28th of July 2021.
Players have been able to use the Online Lounge feature of the Nintendo Switch Online app to arrange Splatoon 2 online battles using their smart device, such as by sending a URL link that allows social media friends to join a room.
As of July 28, 2021, the Nintendo Switch Online app will no longer support the Online Lounge feature.
Of note is that you can still play online modes in Splatoon 2, it’s just the app function that’s being terminated.
You can also continue to start a local or online multiplayer game in Splatoon 2 without use of the Online Lounge feature.
Source: Nintendo Life