I did not expect it to come so soon, but at least Nintendo Network lasted 4 years longer than the DS.
If you want to play Splatoon 1 online from that date, you will need to have @pretendo installed on your Wii U.
This does require your Wii U to be modded if it isn't already running the Aroma environment (unfortunately there's no guides for Tiramisu).
To get your Wii U modded, for the best experience, first follow all the steps for Tiramisu on this page (it isn't required but it might be handy to have both): https://wiiu.hacks.guide.
Considering this account is geared towards Splatoon, I won't say much about the 3DS but you'll need to mod it with the Luma3DS CFW. Unfortunately, older 3DS models are currently unmoddable if they're running 11.17.0.50.
Couldn't fit everything I wanted to in a single post but by modding with Pretendo, you get access to Miiverse and Wara Wara Plaza (this was literally announced yesterday) again.
Support for Splatoon 1 is limited according to Pretendo, but either way, it's our only option atm.
I will be making a newsletter post later on, because I feel like a post here is not going to be substantial enough with the 1500 character limit and Pretendo's limited compatibility with Splatoon atm (Splatfests and Public Battles do not work).
If you have a decent knowledge of the Wii U and 3DS, Pretendo does have a GitHub at https://github.com/PretendoNetwork where you might be able to help out.