ASUS just revealed their plans as to which of their phones will be getting Android 6.0 updates, and not all of the products have been deemed worthy enough of Marshmallow flavors. Most of the ASUS ZenFone 2 variants (there are a lot of those) are getting an update sooner or later, but interestingly enough, the first generation ZenFone models will not be pushed to Marshmallow.
The ASUS device list to be given an Android Marshmallow update is quite small, honestly. The list includes the ASUS PadFoneS (PF500KL), the ASUS ZenFone 2 (ZE550ML/ZE551ML), the ASUS ZenFone2 Deluxe / Special Edition (ZE551ML), the ASUS ZenFone2 Laser (ZE500KG/ZE500KL/ZE550KL/ZE600KL/ZE601KL), and the ASUS ZenFone Selfie (ZD551KL). There are a lot of variants in there, technically speaking, but the range for the update seems quite small.
This means that a bunch of first generation ZenFones – specifically the whole gamut of ZenFone 4 variants, the ZenFone 5, and the ZenFone 6, plus the newer ZenFone Go and the ZenFone Max, will not be getting an update. These first generation ZenFones were built on the early generation Intel Atom dual-core mobile chipsets, but they are deemed old enough not to get the update.
A bunch of people will be sad at this list, but that’s how ASUS chose to go. For those who are included in the update list, wait for the notification that should come out soon.