Which nuget contains SecurityStampValidatorOptions

In a dotnet library project I need access to SecurityStampValidatorOptions.

That appears to be part of the "Microsoft.AspNetCore.Identity" assembly, whose nuget package is deprecated.

So I tried referencing Microsoft.AspNetCore.Identity.EntityFrameworkCore, but that also doesn't help. I get

The type or namespace name 'SecurityStampValidatorOptions' does not exist in the namespace 'Microsoft.AspNetCore.Identity' (are you missing an assembly reference?)

(It is definitely in that namespace though, and in my main ASP.NET Core project, it works.)

Where is that class defined?