https://wccftech.com/lineageos-15-0-htc-one-m8-oreo/

LineageOS 15 based on Android 8.0.0 Oreo is finally live for HTC One M8. A free, community built, aftermarket firmware distribution of Android 8.0 (Oreo) that is designed to increase performance and reliability of stock Android on your phone, this is an unofficial firmware for HTC One M8, updated just last month. Follow this guide for all the steps and links to install LineageOS 15 for HTC One M8.