The biggest headline is the new Water System , which allows you to author oceans, lakes, and rivers using a spline-based workflow.
Because 4.26 was the last major version before UE5, many "Render Pipeline" deep dives are still hosted on Patreon or personal blogs. Search for "UE 4.26 Translucency Lighting" to find articles explaining the changes to volumetric fog, which is poorly detailed in the official changelog. unreal engine 4.26 documentation
The official documentation is organized into several pillars. Below is a breakdown of the most vital sections relevant to the 4.26 feature set. The biggest headline is the new Water System