I received this notice from QB a month ago, "This app will stop updating after April 20, 2021". Why did I receive this message? Thank you!

Hello there, iag1995.

The QuickBooks Online (QBO) mobile app will still work on your android device. You can use it to perform certain billing tasks and create invoices as usual while you’re in Court. There is no further update for the QBO app. Thus, it will still be usable.

What's affected by the said update are the QBO Windows and Mac desktop apps. 

You can also check available features on the mobile app. 

If you have further concerns with the QuickBooks Online app, feel free to leave a comment below. I'll be around to help you more. Stay safe always.