If the drivers were upstreamed it would be supported by the latest kernel.org kernel even before release.
AFAIK outside the Pinephone and Liberem 5 no hardware manufacturers explicitly target this and only 10 year+ old Qualcomm (other vendors such as Freescale tend to behave much better) SOCs have open source graphics drivers because the SOC vendors themselves often refuse to support their own hardware.
Google is able to do this because they build their own SOCs (probably because they got tired of being jerked around by Qualcomm) but still don't merge their stuff upstream (or at least they don't last I checked.)
AFAIK outside the Pinephone and Liberem 5 no hardware manufacturers explicitly target this and only 10 year+ old Qualcomm (other vendors such as Freescale tend to behave much better) SOCs have open source graphics drivers because the SOC vendors themselves often refuse to support their own hardware.
Google is able to do this because they build their own SOCs (probably because they got tired of being jerked around by Qualcomm) but still don't merge their stuff upstream (or at least they don't last I checked.)