The GL based compositing adds alot of complexity and was only ever well tested on closed stacks. It also only supports GLES3.x and still relies on sw_sync timeline which is now a debugfs feature. Those are just the known issues.
Removing the GL compositor means everything related to squashing layers and pre-compositing can be removed. The planner is left as it may be useful when adding back support for overlay planes. With this change, only a single plane is supported until ValidateDisplay learns to do atomic modesetting test for overlay planes.