Like @ThinhDinh said, all features of the app should work the same with anonymous emails. Glide still links the anonymous email to the real email. The only difference is what you see as the developer and what any other users may see if you are displaying those emails in the app. Unless you are trying to send emails, or publicly display all user’s personal email addresses, you should be fine and almost everything else should work the same. I personally think it’s a good security feature to protect your users. I try to use anonymous emails whenever possible because it’s way to easy to obtain a user’s personal details by inspecting the data that’s downloaded to a user’s device.
How old are your apps? If they existed before the change to anonymous emails, then they may have been granted special permission to still get real emails. When you upgraded your plan to a basic plan, then you no longer had that special permission.
Based on David’s post, this mandatory change only applied to new apps. But as I understand it, once you update your plan, you are essentially under a new contract and have to follow the new rules. If you had upgraded to the Basic plan prior to the anonymous email change, then you probably would have been fine. As long as you didn’t need any major changes to your app, Glide allowed you to keep the paid feature for free. Now that you require major changes to keep your app running, you unfortunately need to pay to use them.