Project Treble was announced last May, and its main purpose is to help increase the speed of sluggish software updates that plague so many Android phones. Devices that ship with Oreo and later are required to have it, but a phone that shipped with an older version and was then updated to Oreo isn't required to do so.
In addition to the Galaxy S8/S8+, we also saw OnePlus choose to not support Treble with the OnePlus 3, 3T, 5, and 5T.
This may sound like a huge blow to the Galaxy S8 considering that Samsung's update speed has been notoriously slow, but we still haven't seen any real-world benefit from Treble. The idea of having faster software updates is undoubtedly nice, but until Google can prove that it makes a meaningful difference, it's not worth getting worked up over a phone not supporting it.
If you're the owner of a Galaxy S8, is the lack of Project Treble a big deal to you?
Source : https://www.androidcentral.com/oreo-update-galaxy-s8-does-not-support-project-treble