Answer
A deleted Nintendo Network ID cannot be used again to create a new account. Once it is deleted, that username will not be available anymore. The Nintendo Network ID should only be deleted if you want to discontinue use of the account or do not wish to access all previously purchased games on either Wii U or 3DS.
How to Delete your Nintendo Network ID!
SM127 Vlog: How to delete your Nintendo Network ID
A deleted Nintendo Network ID cannot be used again to create a new account. Once it is deleted, that username will not be available anymore. The Nintendo Network ID should only be deleted if you want to discontinue use of the account or do not wish to access all previously purchased games on either Wii U or 3DS.
Select Nintendo Network ID Settings. Select Other. Select Delete Nintendo Network ID. If you are certain you want to delete your NNID, select the bottom option (I want to delete my NNID & all funds, software licenses related).
To use a Nintendo Switch, each user must have their own Nintendo Network ID. If they don’t, they will not be able to enjoy the online features of the system.
To access the Nintendo Network ID Settings, scroll down to Nintendo Network ID in the main menu of your Wii U console. Select Profile Settings from the drop-down list and update all relevant information for your child’s settings. Then confirm by clicking on Done.
From the HOME screen, tap on Friends on the left panel. Your Nintendo Network ID will be displayed in orange lettering, in the upper-right corner. You can also view your Nintendo Network ID if you click on your profile to open more details about yourself and access functions such as edit your friend code or adjust gameplay settings.
After switching over to Wii Mode on your Wii U console, simply create an Internet connection using your Nintendo Network ID for the first time. If both devices are connected together via a LAN cable and there is already data linked from a previous Nintendo 3DS family system previously on the system (i.e., Save Data Transfer Tool), you may see a prompt asking whether you would like to copy data saved on the other console to the Wii U. If not, you can simply copy data from one system to another using Save Data Transfer Tool on your Nintendo 3DS family system and then launch the application again after switching over to Wii Mode on your Wii U console.
Once an account is deleted, it cannot be retrieved. Once you delete your Nintendo eShop account, any remaining funds in the balance will be lost. You should also remove the original card or code that was used to purchase this content before deleting your Nintendo eShop account to help prevent unauthorized access to it. Note that accounts may be created separately for each device so if you already have an account on Wii U, you will need to create a new one on Nintendo 3DS.
If you linked your Club Nintendo and Nintendo eShop Accounts before the service ended, then you may see occasional offers for select digital games that were offered as part of previous Club Nintendo seasonal campaigns. These games are yours to keep, even after membership ends in September 2015. However, please do not purchase these games from the eShop individually unless you have reached a certain point in a specific campaign; otherwise, purchases made beyond this point will not count toward unlocking additional rewards. The list below contains all potential reward options: