App Tracking Transparency How does effect my exsting app? : IDFA iOS14

The firstly, i want to talk about the IDFA:

    The Identity for Advertisers (IDFA) is the individual 
and random identifier used by Apple to identify and measure iOS user devices. 

Bellow IOS14, Every AdNetworks use IDFA for defund a specific user, then They use IDFA to be used to deliver personalized ads to user. so IDFA help the AdNetworks can show related ads to our users.

In IOS 14, the IDFA is hidden and you and adnetwork can't get this IDFA, You must to ask user to allow tracking permission to continue use IDFA in IOS 14 system.

So my answers are:

Question 1: What's happen if I don't upgrade these apps as per the last WWDC guidelines? 
Is app continue ads serving to the user?

The short answers is YES, Your app still continue ads serving to the user. BUTTTTTT: the Adnetwork will do not know anythings about your user, so all ads will be random and unrelated ads

-> clickRate will be reduced -> eCPM will be reduced -> Your revenue will be down too.

Question 2: What's happen if User doesn't give tracking permission to the app?

Like the my answer 1 when user don't allow tracking permission, you can't get IDFA then Your revenue will be down again..

But in this case, Apple created an another choose for Us and Networks. that is SKAdNetwork which helps advertisers measure the success of ad campaigns while maintaining user privacy. But Nobody can make sure this API will better current IDFA System can make. So you should enable SKAdNetwork to track conversions in tracking don't allow case. To get maximum profit

Question 3: App update does make any impact on revenue from ads?

If your user allow the tracking permission, everything is OKAY like nothing happen. If not, you have the SKAdNetwork and let pray for SKAdNetwork will work nice like The Apple said.

Question 4: What is Funding Choices?

Funding Choices is the Google'Tool to help you to ask user allow tracking permission. Funding Choices and SDK UMP will create the explainer message alert which will automatically be shown immediately before the "Tracking permission" alert. This is automatically and simple. If you don't like Funding Choices, you can create your explainer message yourself to ask user before the "tracking permission" alert is shown

This is all my knowledges after 3 days researching and working about IDFA, IOS14.....

I hope them can help you something. If i had any mistake, reply here!!!. Thanks

The current version my app (pre-iOS 14) is still showing ads, but ad revenue has HALVED since iOS was released. So yes, it seems ad revenue is impacted if you don't implement App Tracking Transparency (ATT).

I have updated my app WITHOUT ATT and Apple now rejects my app. Even though I share no user data with the AdMob framework, disabled location tracking and IAP tracking for AdMob.

So basically my app does not track anything other than the IDFA used by AdMob and Apple rejects it.

My advice would be to implement ATT if you are using Admob. If you don't do that, revenue will decrease and Apple will reject future app updates.