question

You can make use of ProfileConstraints in your API calls. In terms of registration date, there is a property named ShowCreated. If you called Client API GetPlayerProfile and ShowCreated was set true, the callback will provide the creation date you want. Here is a sample in Unity/C#:

PlayFabClientAPI.GetPlayerProfile(
	new GetPlayerProfileRequest
	{
		ProfileConstraints = new PlayerProfileViewConstraints
		{
			ShowCreated = true
		}
	},
	onSuccess => { },
	onFail => { }
);

GetPlayerProfile onSuccess callback will not provide additional information unless you manually set them true in the ProfileConstraints.