Created attachment 20271 [details]
Small reproduction of the bug.
In TableViews in UWP, Viewcells don't use the CanExecute event. I have a bigger project which invokes the event in a TextCell and ImageCell, but I've created a small reproduction to show the bug (no event invoked, just a hard-coded function that returns false). I have confirmed that the bug does not exist in Android and iOS, at least on my bigger project and in the reproduction. Tested in both Forms 188.8.131.52 and 184.108.40.206-pre4
I will admit to not testing this with any other controls that have a Command Binding such as buttons, or ViewCells in a ListView, and I have not tested earlier releases of Xamarin Forms. I have also not looked into a work-a-round using a custom renderer. If I get some time to work on this though, I will update this thread.
Hopefully my attachment works, it's my first time submitting a bug report.
Thanks for the reproduction; there is a noted difference in behavior between UWP and the other platforms, and I also checked against the latest nightly build to be sure.
Should be fixed on 2.3.5-pre1