In practice the kernel only wants to add support for DTs that conform to a reviewed and approved schema, but most/no manufacturer wants to do the effort of having every single thing on their DT schema bikeshedded to death on the device tree mailing list before the device is manufactured. So the vendor just makes up whatever kind of unapproved DT schema that only their own vendor kernel fork supports, and if mainline support is ever added it will have its own DT.