jimbouk
One of old HTC phones (back in the mid 2000s) had a 2.5mm port. …
I think it might have been more than one. I certainly had an HTC-made smartphone on orange (yes, a proper smartphone, in 2006 ;)) that had a 2.5mm 4 pole socket for the headset. Nothing wrong with it at all, IMNSHO, although that device wasn't really thin enough to need it.
A flexible expanding port that could take both 2.5mm and 3.5mm plugs would be pretty cool, although I suspect the different pin-pitch might make that tricky.
Then again, this whole “need to make the device thinner” shtick is clearly nonsense, because a USB-C port is
maybe 0.5mm narrower than a 3.5mm socket. If you can get a USB-C on your chassis you can get a 3.5mm headphone port. Be honest and admit that you actually want to reclaim that space to squeeze a slightly larger battery in….