Hola,
Copiado de blog Sonymobile.com 15 / 09 / 2017:

Thanks for those of you who have reached out with questions about other devices, notably the Z5 Series. We do not plan to bring Android 8.0 Oreo to devices not included in the above blog post and wanted to explain why.
We aim to support all premium range devices with the latest Android updates for eighteen months after the device has launched. We may choose not to upgrade a mid-range or entry-level device if testing does not meet our exacting standards and we feel the hardware isn’t capable of delivering a stable user experience.
In cases where there may be a serious security risk or vulnerability with a device, we will aim to make firmware patches available, even if the device in question has received its final Android operating system upgrade.



No planean en ningún caso actualizar a Android 8 el Xperia Z5.

https://blogs.sonymobile.com/2017/08...roid-8-0-oreo

Saludos