Planar IX2790 is full 5K (2880 tall) but only 27 inches but that is the proper size for a 5K Retina display. It's not Thunderbolt so it is only supports 8bpc instead of 10bpc at 5K.I've checked all possible display so far, bigger than 27". Most of them are curved and none of them is real 5K. There is only one 8K.
Displays with higher resolution result in tiny fonts, not really usable for me.
Maybe the biggest problem of retina display is the TB3 connection which none of the PC-GPU has. Therefore there are no retina displays on the market. Maybe we'll see one from Dell, as their 5K isn't available anymore and no-one can run the 8K version.
Other 5K displays may be 2160 tall or 1440 tall. They are larger but have fewer pixels which means they are not meant to be retina displays.