They sure lounged around when it came to making more games use it.
In an announcement on the Nintendo Switch Online app, as well as the official support page, the Online Lounge feature is being discontinued for Splatoon 2. Players will still have the option to utilize voice-chat, but the ability to send URL links to friends and create lobbies is no more as of July 28th, 2021. This is likely due to the release of Splatoon 3 in 2022.
Nintendo's full statement: "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. However, the app will continue to allow friends to voice chat, and no other changes will be made to the app at this time.
You can also continue to start a local or online multiplayer game in Splatoon 2 without use of the Online Lounge feature."