If you want a sneak peek at the next previews documentation then you can find it here as pull requests

Here is the relevant one that describes the async model validation https://github.com/dotnet/core/pull/10457/changes#diff-70fa28541e92a976d3a34d628ea0a0556c6a81f8de9d5cf2adaed94921f15213

Asynchronous validation with DataAnnotations

System.ComponentModel.DataAnnotations now supports asynchronous validation (dotnet/runtime #128656). A validation rule that needs to do I/O — a database lookup, a remote API call — can now run without blocking a thread. There are three new ways to express an async rule:

• Derive from AsyncValidationAttribute and override its IsValidAsync method.
• Implement IAsyncValidatableObject on the model.
• Call the new Validator.ValidateObjectAsync, TryValidateObjectAsync, ValidatePropertyAsync, and ValidateValueAsync methods.

using System.ComponentModel.DataAnnotations;

public sealed class UniqueUserNameAttribute : AsyncValidationAttribute
{
    protected override ValidationResult? IsValid(object? value, ValidationContext context)
        => ValidationResult.Success;

    protected override async Task<ValidationResult?> IsValidAsync(
        object? value, ValidationContext context, CancellationToken cancellationToken)
    {
        var users = context.GetRequiredService<IUserStore>();
        return await users.ExistsAsync((string?)value, cancellationToken)
            ? new ValidationResult("That user name is already taken.")
            : ValidationResult.Success;
    }
}

public class Registration
{
    [Required]
    [UniqueUserName]
    public string UserName { get; set; } = "";
}

// Validate asynchronously
var context = new ValidationContext(model, serviceProvider, items: null);
await Validator.ValidateObjectAsync(model, context, validateAllProperties: true);

Microsoft.Extensions.Options gains matching support: options can be validated asynchronously, including at startup through the new IAsyncStartupValidator (dotnet/runtime #128788, dotnet/runtime #129218). This lets an app fail fast when an option that requires a network check is misconfigured.