The async keyword is not actually part of the inherited method signature, but more of a signal to the compiler that it needs to compile and rewrite the method according to the pattern for async methods.

As such, you can leave out the async keyword on inherited methods if the inherited method does not use the await keyword.

Note that you will still have to return a Task or Task<T>, that part is part of the inherited method signature.

So this will give you a warning:

class Base
{
    public virtual async Task<int> Method()
    {
        await Task.Delay(10);
        return 42;
    }
}

class Derived : Base
{
    // next line produces warning
    public override async Task<int> Method()
    {
        return 42;
    }
}

The warning is this:

Warning: CS1998 This async method lacks 'await' operators and will run synchronously. Consider using the await operator to await non-blocking API calls, or await Task.Run(...) to do CPU-bound work on a background thread.

This, however, will not produce a warning:

class Derived : Base
{
    public override Task<int> Method()
    {
        return Task.FromResult(42);
    }
}

Note that I changed the return statement in that last method because part of the "magic" that the async keyword brings is to automatically wrap the return value inside a Task<T>. If you have other ways of obtaining a Task<T>, obviously you do not need to wrap the result like I did above.