In recent years, OnePlus has taken seriously updating its devices. While it may seem that the stable versions of OxygenOS take a while, the Open Beta program represents what OEMs should do: provide those who are willing to have early access to new features, Android versions, and more. In addition to trying to reinforce its positive reputation in this area, OnePlus launched the Software Maintenance Program, which promises two years of software and three years of security versions for each device and is already in effect.
The OnePlus 3 / 3T is already running the two-year mark, or close enough in the case of the latter, but OnePlus has said that those phones will receive an Open Beta more, which will be followed shortly by a new release stable to return them to the regular channel. November will mark the last update of functions, with another year of security patches to follow.
From there, the OnePlus 5 / 5T will get another year of software and two more security updates and the 6 still has its full release release the calendar to go. None of this is new or frankly surprising, it's what you get with the Pixel and Android One devices, but now we have documentation that OnePlus is committing to the two-year / three-year launch schedule. He also said that the launch dates of variant T would be considered in this List.