SUMMARY I think it would make sense to add this toggle for the same reason that the Bin Effects toggle exists. It can be extremely tedious to disable per track effects when there are large numbers of tracks. Track effects are a good way to reduce cpu usage for things like color grading, sharpening, etc that aren't needed during regular editing. I don't think this would necessarily require a new menu item. It could just be exposed in the Keyboard Shortcuts as an optional action or perhaps provided a shortcut by default if desired.
(In reply to James Roberts from comment #0) Something like https://docs.kdenlive.org/en/tips_and_tricks/tips_and_tricks/disable_all_timeline_effects.html ?