Ambient shadows

Screen Space Ambient Occlusion (also known as SSAO or ambient shadows) was a Roblox Studio feature released on January 28, 2011, along with Updated Graphics. It could only be seen on max graphics settings. Mac support for ambient shadows was supposed to be implemented by Summer 2013, but was canceled when ambient shadows was replaced with Dynamic Lighting in 2013.

Removal of the Screen-Space Ambient Occlusion [Ambient Shadows]
It's believed that when Dynamic Lighting was released, ambient shadows were removed to encourage people to use the new lighting system. As of December 19th, 2020, there is no confirmation on why ambient shadows were removed.

As of January 9th, 2020, Roblox had implemented HBAO (Horizon-Based Ambient Occlusion) to make the render look better, as it is an improvement to SSAO, however it's been replaced with SSAO due to noticing bugs. Later in January 29th, 2020, it is been reimplemented after fixing issues

Trivia

 * Roblox would automatically turn off this feature if it detects the user receiving less than 30 FPS because of the shadows.