Source 2 engine
Although the Source engine was designed by Valve to be modular and updateable - which it was throughout its lifetime - work was eventually begun on a new engine: the Source 2 engine. The reasons for leaving the Source engine behind and creating a new engine were likely due to technical debt - in an interview with PC Gamer the Counter-Strike 2 dev team said that continued Counter-Strike: Global Offensive "development was fundamentally limited by aging technology" and that "Better tools were the foundation of Source 2" (Stanton, https://www.pcgamer.com/counter-strike-2-interview/).
It is not clear when work began on the Source 2 engine but the Alien Swarm SDK was released on 19th July 2010 and src/public/tier0/logging.h made reference to the Source 2 engine though it had likely been in development for some time before 2010. In August 2012, further references to Source were found in Source Filmmaker. In November 2012 Gabe Newell revealed in an interview that a new engine was in development (Benson, http://www.pcgamesn.com/gabe-newell-confirms-source-engine-2-has-been-development-while-valve-are-waiting-game-roll-it-out).
DirectX 11
The Source 2 engine ships with a native DirectX 11 renderer and the engine was designed with DirectX 11 in mind (Ginsburg, https://www.khronos.org/blog/reducing-draw-time-hitching-with-vk-ext-graphics-pipeline-library).
Rubikon
The Source 2 engine was built with a new physics engine to replace Havok which was the basis for VPhysics. Rubikon was built in-house by Valve rather than being developed using third-party software.
Vulkan
The Source 2 engine ships with a Vulkan renderer. Source engine Vulkan support was through dxvk but the Source 2 engine has a native Vulkan renderer.
ScaleForm
ScaleForm was brought along to the Source 2 engine as part of moving Dota 2 to the Source 2 engine, but it was replaced by Panorama on November 9th 2017 (https://store.steampowered.com/oldnews/34477). Presumably ScaleForm was kept as part of the initial Source 2 engine transition in order to reduce the complexity of the change of Dota 2's engine.
32-bit support
Dota 2 (Source 2) shipped with 32-bit support but it was removed in 2021. Currently SteamVR Home is the only Source 2 engine game that still has 32-bit support.
List of games using the Source 2 engine