NormalizedUserName VS Username in DotNet Core

1) Normalization stops people registering user names which only differ in letter casing.

2) No - those fields are part of the basic data model.

2. Yes, you can (if you really want):
   1. FindByNameAsync should search by u.Name case-insensitive (do you trust your DB settings?)
   2. GetNormalizedUserNameAsync should return user.Name.ToUpperCase()
   3. SetNormalizedUserNameAsync should do nothing

Please note that 2.1 can skip any index on name column in DB and hurt performance of your app (check your DB, again). Or cause 'client-side' execution (and again, dramatically hurt performance). Depending of your implementation.

I use such "minimized" User class only in internal enterprise systems which uses only specific OAuth provider and accepting users only from specified domain (Google Apps). This systems does not perform any search by user name and I safely throw NotImplementedException in many methods.