It was announced during the preview of asp.net core 2.1 that the Identity UI would be moved to a new Razor Class Library.
https://blogs.msdn.microsoft.com/webdev/2018/03/02/aspnetcore-2-1-identity-ui/
It is still possible to scaffold the Identity Views into your own project if you prefer local views:
https://docs.microsoft.com/en-us/aspnet/core/security/authentication/scaffold-identity?view=aspnetcore-2.1&tabs=visual-studio