because slow ring is only for more stable builds and currently there is no a really stable build of 19H1 yet.
It is on WU, see the WU pics at the OP, bottom spoiler. You can check this: https://forums.mydigitallife.net/th...ast-current-rs_prerelease.78784/#post-1496060 You can try to edit the regkey, @abbodi1406 mentions.
Here created Windows.old, upgraded from 18309. And night light is not working here, I saw someone here complaining with this.