3ds Max 2019 introduced support, which significantly improved in the 2021–2023 updates. You can find free "OSL Gobo" shaders online.

Leverage the and the improved Viewport Shading . You can now drag and drop your dappled light texture into an Arnold Light and see the results instantly, making the "trial and error" process much faster than it was in the 2018 version.

Because these are actual 3D instances, the shadows (dappled light) change accurately as the "sun" moves throughout the day using the 3ds Max Daylight System. 5. The "Better" Free Alternative: OSL Shaders

You can slide the "Density" and "Offset" parameters. This acts as a real-time dappled light generator, allowing you to tweak the softness (blur) of the leaf shadows without moving the light source. 4. Dedicated Plugins: RailClone or Forest Pack