I think, they deleted SuperSU because some idiots already bricked some consoles by wrong using of root. When root is preinstalled from factory it means that using of this root do not avoid Guarantee.
So, they don't want to accept bricked by users devices.
I think that someone claimed on GPD and wanted full refund for a bricked (with preinstalled root) device.
That's why they removed root and from now they can say that your root and your brick - is only your problem. No warranty on root-bricks. Its logical.