The ASUS ZenFone 4 Max only received an update to Android Oreo back in October, but the company is hard at work with keeping the phone updated. You can now download an AOSP beta without ZenUI for the device with the model name ZC554KL.
This beta is more disruptive than we're used to from Pixels, as it will remove all personal data from your device. Thus, make sure you back up your phone before installing the pre-release software. It will also replace ASUS's custom ZenUI with Android's stock interface, so if you prefer the company's own design, you probably shouldn't install this software. Keep in mind that the beta won't get routine updates either, so you have to jump through some hoops to get back to stable.
ASUS was long known for being bad with updates, but lately, the company is putting some steam behind getting new releases to us. Its ZenFone 5Z is even one of the few devices to receive the Android Q Beta, so there's hope on the horizon that the company keeps up the good work.
The ZenFone 4 Selfie is following in the footsteps of the 4 Max with a Pie beta of its own, and it's also based on AOSP. Both the ZB553KL and ZD553KL models are supported and the same warnings explained above apply: the beta will wipe out your device and you won't get OTA updates.