Deep within Wii U system update 5.5.2, the following text has been unearthed:
The Miiverse service has ended. Miiverse and any software features that make use of Miiverse are now unavailable.
Nintendo made quite a song and dance about Miiverse back in 2012, and some even claimed it could become a social network for gamers to rival the likes of Twitter and Facebook. However, the fact that Miiverse isn't supported on Switch is a pretty sure sign that the company is walking away from the service, and this message seems to confirm that. It's surely just a matter of time until the plug is pulled entirely.