How can I disable the deprecation warning on the line where it is handled?

It's a bug in the compiler, vote on it: 

forums.garmin.com/.../sdk-4-2-0-beta-1-won-t-compile-appbase-getproperty

https://forums.garmin.com/developer/connect-iq/i/bug-reports/set-getproperty-deprecate-on-sdk-4?CommentId=b16fd27e-39a3-450e-8e03-812435f75c8f

And a few more... Just search the bugreports

From the 4.1.7 release notes:

• Do not error when using -l 3 and the warning is due to using a deprecated function.

And from the 4.2.0-beta 2 release notes:

• Change deprecation messages from errors to warnings.

The code still shows a warning, even though it can be compiled. Is there a way to write code supporting old devices without getting this warning?

Which SDK are you using?

Just updated earlier today, v 4.2.3 I believe 

yes, Garmin didn't fix this. I doubt they will, and if they will then I hope they fix the real bugs before this. But I agree it's annoying.

What are you seeing the warning about?  What's the waring you see when compiling?

using Toybox.Application as App;

class Properties {

  function getValue(key) {
    if (App has :Properties) {
      return App.Properties.getValue(key);
    } else {
      return App.getApp().getProperty(key);
    }
  }

  function setValue(key, value) {
    if (App has :Properties) {
      App.Properties.setValue(key, value);
    } else {
      App.getApp().setProperty(key, value);
    }
  }
}

\Properties.mc:9,6: '$.Toybox.Application.AppBase.getProperty' is deprecated.

\Properties.mc:17,6: '$.Toybox.Application.AppBase.getProperty' is deprecated.

Both when compiling for Fenix 3 and Epix 2, or any other device

From what I see, your code doesn't have any typechecking code.

Try turning off typechecking, either in the Monkey C extension to VS Code, or by adding the following line to the project's monkey.jungle

project.typecheck=0

Or you can just ignore the warning.

I have apps that do basically the same thing you are doing, and with the line in the monkey.jungle file, I don't see the warning.