There was CyanogenMod Theme Engine (CMTE) before Substratum. Long ago, CyanogenMod 9 added system-wide theme capabilities to CyanogenMod builds. As CyanogenMod 14 came, CMTE was nowhere to be found. Meantime many other custom ROMs started to favor the OMS-based Substratum Theme Engine in their builds. Even, now Android 8.0 includes native OMS support. It seems that the Lineage OS team has pulled the plug on CMTE for good.
Some time back, we heard CMTE was dead but there was still a ray of hope by some users that it would return in Lineage OS. It did not happen, Lineage OS 15 came and CMTE and remaining theme engines were in the procedure to remove from the source. This happened on 12th October and symbolized the death of CMTE.
Substratum has risen very much in these years and their theming framework has got better since Sony moved from Runtime Overlays (RRO) to Overlay Manager Service (OMS). Substratum has taken the place CMTE for custom ROMs.
You can theme your Android Oreo device as it includes native support for OMS. It means you can use Substratum without any root on any device. It can be used by most of the custom ROMs which support OMS including the Lineage OS 15. Now, it is possible that the Lineage OS team will develop their own theme engine.