OnePlus has started rolling out OxygenOS 5.1.5 to the OnePlus 5 and OnePlus 5T, and it seems like the company has enabled Project Treble support with this update. In the company's official changelog for the update, it is listed that this update brings Android security patch for August 2018 to both devices, while it also enables OnePlus 5 and 5T users to confirm their PIN without taping '√' to unlock the phone, but it doesn't mention anything about Project Treble support.
That being said, the company has enabled Project Treble support in OxygenOS Open Beta 13 and 11 updates for the OnePlus 5 and 5T, and the feature is now silently enabled in OxygenOS 5.1.5, though it's rather interesting that the company did not mention that in its official changelog. Project Treble, for those of you who do not know, makes it easier for developers to update devices, and that's actually applicable for ROMs as well. What makes things even more interesting, is the fact that OnePlus actually said a while back that its older devices, including both the OnePlus 5 and 5T, will not be getting Project Treble support, but it seems like the company changed its mind somewhere along the way.
OxygenOS 5.1.5 update for the OnePlus 5 and 5T seemed rather small at first, without any notable additions, but Project Treble changes things, definitely. In any case, the company has started pushing out this update to both devices yesterday, though do keep in mind that this is a staged rollout, as the company does not want to push it to everyone straight away, just in case bugs appear. VPN will not help you download this update, as it is not region-based, it is rolling to a small number of users (randomly), and after a few days, after OnePlus is sure bugs are not present, it will roll out to everyone. All in all, most of you will probably have to wait a couple of days in order to get this update.