"You must have account hold and restore implemented in your app by November 1, 2020." -what does this message mean in Google Play Console?

As mentioned in Android Developers Blog,

Account hold is a state the user enters after a renewal fails due to a payment issue. During account hold, the user loses access to the subscription while Google notifies the user and retries the payment method.

Restore enables users to resume auto-renewals after they have canceled the subscription but before the subscription expires.

So you have to make changes only if you have subscriptions in your app and using Google Play Billing to manage them.

!! Please notice that in the new payments policy, Google enforces the app providers to use Google Play’s billing system if they are selling subscriptions, virtual items, app features, or cloud services.

After a long, long time of searching, I finally found an official answer from someone affiliated with Google (provided two days ago).

"If your Android app sells subscription products, then you will need to make sure these subscription features (account hold, restore, pause, and resubscribe) are supported. If your Android app does not sell subscription products, you will not need to worry about this upcoming change."

The answer is here: https://issuetracker.google.com/issues/170592154

Whether you have a paid app or in-app purchases or you use any external payment gateway you will not face any issues unless you have any subscription model from Google Play Billing.