Can’t find Desktop Grid settings in Plasma 6.
In Plasma 5 they were in System Settings → Workspace Behaviour → Desktop Effects under title Window Control (or Management).
In Plasma 6.4.3 they are not anymore. Desktop Grid itself is in Plasma 6 (available in Mouse & Touchpad → Screen Edges).
In Plasma 6 other desktop effects are under Window Management.
(By the way, Screen Edges settings are in a totally wrong and illogical place under Mouse settings - instead they should be in Display & Monitor)
Picture: Plasma 5 System Settings