The HD Cache GI solver is deprecated - it can cause flickering, and will be removed in the future. We recommend switching to the new UHD Cache GI solver and re-saving the scene.
This message occurs when you have the Secondary GI Solver set to HD Cache. This is an old, deprecated solver, which provided worse quality output, especially in animations.
Solution: Switch Secondary GI Solver to the new UHD Cache in the Render Setup > Performance tab.
See: Using the UHD Cache