remove annoying critical warning in lower left corner

There's no way to remove it, as far as I know.

I understand that you don't want to see the in-product warnings on your end, mechman.

Let me share some insights about the warning in QuickBooks Desktop.

The system notifies you in advance about the services for older versions of QuickBooks Desktop that are scheduled for discontinuation. You can read this article to learn more details: QuickBooks Desktop service discontinuation policy.

Moreover, the option to remove the warning is unavailable.

If you have any other QuickBooks-related concerns, you can post them anytime. The Community is here to help.

That's an terrible answer. 

This company disables portions of the software to try and make people update and spend 100.00 of dollars over and over. 
Make the option to remove that a thing. Pathetic. 

@GlinetteC 

RE: The system notifies you in advance about the services for older versions of QuickBooks Desktop that are scheduled for discontinuation.

Well, it's no longer 'in advance'. It's now 'old news'. I still use QB 2017, since the newer versions are relatively unstable and Intuit fixes few bugs these days. It's still hyper-actively warning me about an end of service that happened years ago.

It's almost as amusing as the windows I get when updating client files from an older version to a less older version, like QB 2019.  During that process QB proudly announces "You've upgraded to the latest version of QuickBooks!"  If course, it hasn't been the latest version for many years. This sort of messaging in QuickBooks reveals a staggering lack of logical thinking from the developers.