It appears that this new plugin makes it mandatory to validate user subscriptions server-side. When implementing the plugin, I had to create a back-end service that provided the expiry date back to our app since we couldn't use SubscriptionManager to grab that information anymore.
I can't really recommend a specific way of doing this, because everyone's back-end will be different. For us, we utilize Docker containers on DigitalOcean droplets that our app and database can communicate with. This allows us to have a centralized location for back-end services, which we write in Python using Flask.
We have set up one that can go through our database, find every subscription that has expired based on the saved DateTime, and validate whether it has renewed or not. We added an extra endpoint to that service for grabbing the expiry date of a Google Play subscription, as mentioned above.
Subscription information can be obtained by accessing the Google Play REST API's purchases.subscriptions.get. This will return a SubscriptionPurchase object, which provides relevant information that you can then process to find out attributes such as is_subscribed, is_expired, etc.
It may be possible to send this directly from your Unity app/game, however this may also make man-in-the-middle attacks possible (admittedly my knowledge in this area isn't quite there, so I recommend you do your own reading on this).
Also as just a general suggestion, I recommend you try to post questions across both here and the UnityIAP sub-forum. The UnityIAP support folks are super active and even if they can't help because this is a Google implementation, it should definitely put it on their radar! I try to post there whenever I can as it allows them to make improvements to UnityIAP. (:
