I also suspect that most people might notice letterboxing at first, but then just put up with it and stop caring or really even appreciating it. If this is true of things like motion smoothing, which is even more obvious, then it’s probably true of letterboxing.
I’m having this argument in my gamedev meetings rn, so I feel like I have a lot of stake in thinking most people just don’t think about this stuff; and it’s even less of a concern if you believe in the work you’re producing, that it will be engrossing to viewers/players.
Letterboxing is probably invisible after a certain point, but the effects that deliberate framing and whatever other affordances it offers can produce shocks that are satisfying in an immediate, emotional sense but also on the aesthetic level, surviving critical reflection.