(I haven't been able to find that discussion searching all the usual places, I'm positive I saw it somewhere a couple years ago).
There are many more functions in futures than just poll_fn this affects, every function which takes a closure does not structurally pin that closure since it's pointless. As an example there was this previous discussion thread
Will std similarly change all these to inherit their Unpinness from the closure when uplifted (and promise to pretend it's a FnPinMut and can internally pin its moved enclosure)?