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.
That's probably because High Sierra introduced AirPlay2 (before, you were running AirPlay). With AirPlay2, it's no more a system wide output that's being redirected to your remote player, but instead a per-application output. Technically speaking, it's now a userspace library that application must implement.
Since only iTunes supports AirPlay2 for now, I ...