Android's Project Treble not coming to most currently available phones?
Well, according to a OnePlus representative, in order to implement Treble, the device needs a storage partition dedicated to whatever vendor files need to be preserved and separated from the Android framework files. Trying to add that partition via an OTA update to existing devices right now risks to cause errors and even maybe "brick" some of said devices. Which is why OnePlus is staying away from that idea for now.
Will it launch the OnePlus 6 with Treble support? It's still unknown. The company was quick to promise that its software team is dedicated to working hard on delivering timely updates for the "flagship killers".
As for other manufacturers and their phones — if their devices didn't launch with the needed partition pre-made (and they probably didn't), it's highly unlikely they would be willing to try and create one on existing phones via a simple OTA update. It's a risky ordeal and it requires an extra amount of work. Right now, the Essential Phone, the original Pixel and Pixel XL, and the Huawei Mate 9 have been updated to support Treble. Others seem to be making the move to Oreo without putting the effort into creating that extra partition. HMD has confirmed that its Nokia phones will get Android 8 but without the "Treble partition". Samsung's Galaxy S8 got the new update, but also doesn't have Treble.
Treble requires 2 conditions to be met:— Juho Sarvikas (@sarvikas) December 5, 2017
1) Adapt to the new Vendor Interface implementation
2) Standalone Vendor partition to contain Vendor Implementation
Nokia 8 does not have separate Vendor partition in ROM space and the partitioning cannot be done OTA.
So, unless the trend changes abruptly, it seems that most 2017 phones won't be part of project Treble. Oh well, here's to 2018!
source: OnePlus forums