Every Manufactures are taking several months to update their mobiles to their newer version, so google introduced Project Treble a feature of Android Oreo and later that makes OTAs modular. Devices that launch with Oreo will support Treble, but those updated to Oreo won’t—at least mostly.
Now OnePlus has confirmed in their forum that their current OnePlus devices will not support project Treble. OnePlus held an AMA on its forums and explained the Treble situation further. Apparently, adding the necessary partition to phones in an update comes with a high risk of bricking devices. Thus, OP had decided not to risk it.
Project Treble separates the vendor implementation (device-specific, lower-level software written by silicon manufacturers) from the Android OS framework via a new vendor interface.
In Android 7.x and earlier, no formal vendor interface exists so device makers must update large portions of the Android code to move a device to a newer version of Android:
With Treble, a new stable vendor interface provides access to the hardware-specific parts of Android, enabling device makers to deliver new Android releases simply by updating the Android OS framework—without any additional work required from the silicon manufacturers.
Project Treble requires a storage partition, by which the Android framework and vendor image are separated. However, because partitions were not required of Android N and previous versions of Android, all of our current devices do not feature a partition. According to our tests, if we were to modify the partition layout via OTA there is a risk that devices will brick during the partitioning. We feel this poses too great a risk for our community of users, which is why we have decided not to implement Project Treble on current OnePlus devices.