Project Treble won't be included on any existing OnePlus phones according to the company. This information comes from OnePlus through the means of AMA-style post on its own OnePlus forums where it's giving the users in its community a chance to ask questions about its current lineup of devices or really anything that they might have weighing on their minds. One such question was whether or not OnePlus would be including Project Treble technology with the Android Oreo update that is supposed to hit devices sometime within the next few months.
According to OnePlus, none of its current phones, including the OnePlus 5T which just launched, will have Project Treble included as part of the Oreo update once it's pushed out to users. For the OnePlus 5T, that's a version of the software which is supposed to begin rolling out to users of the device by end of this year in e beta form, and for the OnePlus 5 (and perhaps prior devices) the Oreo update is supposed to land sometime in the beginning of 2018. That might not seem like a long time and the lack of Project Treble support may not seem like such a big deal to some users, but for anyone who isn't keen on having to wait for updates longer than they need to, this is where Project Treble becomes a useful tool for developers and OEMs to have at their disposal, as it's main goal is to make it a quicker and simpler process for both groups to push software updates out to devices and be able to do so for a longer period of time.
With this in mind, Project Treble support would ensure that future major software updates would be able to reach both OnePlus 5 and OnePlus 5T owners in a timely manner and perhaps around the same time, while also reaching users at a rate that isn't too far behind that of Google. If faster updates are something you are wanting in a device, it might be worth considering waiting for OnePlus next smartphone as this should have Project Treble included. OnePlus also doesn't seem to give a reason for why it isn't adding this technology as part of the Oreo update that it will be pushing out to its phones.