Proposing to call out the following:

For iframes: Current permissions policy that makes use of 'allow' attribute should be followed for clipboardchange event.
Listening to "clipboardchange" event within a cross-origin iframe can be enabled with Permissions Policy which allows for selectively enabling and disabling specific browser features and APIs. Specifically, "clipboard-read" permission needs to be passed to the "allow" attribute of the iframe, similar to how this is required for accessing async clipboard read APIs within a cross-origin iframe.

For fencedframes:
Given that the fencedframe element is a type of iframe with more native privacy features built in and only limited permission are considered for fencedframe via allow attribute (more information) proposing to not fire clipboardchange event in a fencedframe.