Just like that, without any notice or warning, Daydream no longer works on my S9+.
The same applies to S9 and Note 9, which used to be perfectly compatible with Daydream, but now all you get is this:
Note that the scene behind this dialog window is actually perfectly rendered, and the phone itself is using the "low motion-blur" screen mode.
The problem is, in Android 8 and Android 9, these phones have defined a system flag "android.hardware.vr.high_performance", which is no longer defined on Android 10.
This is a Samsung-specific problem, as Pixel phones running Android 10 can still use Daydream perfectly (as reported in this Reddit thread).
Hopefully Samsung will be able to get around to fix this problem in a future system update, as it only requires them to add a single xml file to the system image...