Are AirPlay2 device retrocompatible with AirPlay I've a 2012 MacBook pro running MacOS 10.11. It does not have AirPlay 2 (but AirPlay is running fine).

I wonder if I buy a AirPlay2 device if they are going to work with it ?
AirPlay 2 is backward compatible.  You can use AirPlay 2 to play to any AirPlay compatible device.

The main difference, but not the only difference, is that AirPlay is 1:1; you can play to only one device.

AirPlay 2 is 1:many; you can play from one device to multiple other devices.

Here's a good write-up on AirPlay 2 and the benefits it brings.