A major criticism of Android phone makers since the launch of Android Oreo has been a failure to support Project Treble, Google's modular upgrade scheme that has the potential to help the custom ROM development in ways that previously weren't imaginable. With Project Treble, A single system image can boot across multiple devices, so it's not surprising that some people have been disappointed that not all OEMs are supporting it. Nokia and OnePlus both claim that their devices don't have the necessary partitions -- a fair excuse, given that Project Treble requires a /vendor partition to hold all of the device Binary Large Objects (BLOBs). But Asus didn't let that stop it from releasing an Android Oreo-based update for the Asus Zenfone 4 that brings Project Treble support without a separate vendor partition.

Now, it isn't the same sort of Project Treble that's come to other devices so far. In fact, it's nowhere near as useful to custom ROM development as, for example, the Huawei Mate 9's Project Treble implementation. Because Asus chose not to use a /vendor partition, the device BLOBS are stored in the /system partition instead, which means it's impossible to create a single system image on the Zenfone 4 that'll boot on other Asus devices. That doesn't mean it won't benefit custom ROM developers, though -- Asus Zenfone 4 device BLOBS have now been standardized, and custom ROM developers can now take the same approach developing ROMs as they would creating a single system image -- they'll just incorporate the binary objects within the system partition instead.

While Asus's Project Treble support for the ZenFone 4 isn't going to help developers create a universal system image for all devices, it's going to help custom ROM development for the phone and make it easier for Asus to upgrade in future.

Update: More Context

For custom ROM developers, Asus' Project Treble support should make things a lot easier in some aspects. While it's true the best parts of Project Treble will allow the same system image to be booted on any device, devices which don't have this partition but still "Treble-ize" their BLOBs, so to say, can still be used on any Project Treble supported version of Android with or without the /vendor partition, so long as the necessary adjustments are made.

Source: ZenTalk Forums