Strata Source

Strata Source (more informally known as Strata, and formerly known as the Chaos Engine) is a third-party Source engine branch using licenced code. It is a modified version of the CS:GO engine branch that combines the features of both CS:GO and the Portal 2 engine branch, along with many improvements that bring Source close to modern standards. This engine branch was first released publicly with the launch of Portal: Revolution, and is still in active development.

Features

Example from Portal: Revolution showing the intricate environments capable with Strata Source

Feature list from the Strata Source Wiki
New since the CS:GO engine branch:

Native DirectX 11 renderer: Support for DX11 by default
DXVK support on Linux for DirectX 11
64-bit executable: Game runs on 64-bit on all platforms; 32-bit and Mac OS support has been dropped
PBR shaders

Introduces a new PBR shader that attempts to create realistic reactions to light

Ported Portal 2 engine branch features: Including but not limited to World Portals and Portal gun
Backwards compatibility with BSP v19-21, as well as BSP v25 support
Backwards compatibility with Source 2013 (and older) branch models: The model format found in games such as is now supported.
WEBM video support, replacing older Bink videos: Increased framerate, lower file sizes, and easier to author
New v7.6 VTF version: Adds support for the higher-quality ATI1N, ATI2N, BC6H, and BC7 texture compression formats, as well as lossless deflate compression on top of existing image format, for reduced size. Better support for 4K textures^[How?].
Many improvements to Linux support
Fully cross-platform engine tools and utils
Steam Audio
Many improvements to the Hammer level editor, named Strata Hammer
Many code quality improvements (Refactors, cleanup, and C++20)
Sentry crash report integration