Feature Usage Sync in Edge

Edge > Settings > Profile > Sync



Why can't I turn that off? Clicking on the toggle does nothing.

Sorry, I tried to duplicate your problem but had no issue toggling it off. I was on Edge 135.0.3179.98. It toggled off fine. I then updated Edge to 136.0.320.50. That option was automatically toggled off again. I was able to toggle it on and off, though.
Firt close and restart Edge to see if you can disable that setting.
Disable any Edge add-ons and try again.

If that doesn't work,the only thing I would know to try would be to reset Edge, but that puts everything back to default (setting>reset settings.)
However that puts Edge back to default. It would delete all your settings, add-ons, history,cookies, saved passwords, and bookmarks. So if you do that make sure you export all that.
If you are syncing Edge, all that might be imported back into Edge after a reset, but I would not guarantee that so I would manually export everything and then you can import anything that is missing after a reset.

Same issue in Windows 11 (2 machines) but not Edge in Win 10 (1 machine) all up to date. I believe this is recent as I only sync Favorites, History and Extensions between machines so I've been in this section enough to know it wasn't this way until recently. Tried logging out, disabling all extensions and a heap of other settings just to test, resetting the Edge app, looking at internal settings and web searching. This is the only result specifically referring to Feature Usage. While it's not greyed out it may as well be.

It was added in version 134 and MS has not polished it yet. Even if you disable it by policy, it still shows as ON.