Although others answered similarly it does not seem they had first hand experience, well, I do. I called the airline, Norwegian in this case and they didn't bat an eye. I got a reservation for two seats, one for say Smith/John (me) and one for Smith/Exst (short for extra seat). I have no idea what they would do if someone with the given name Exst would show up.
This EXST seems to be the practice for Cathay Pacific as well and this document very strongly suggests it is a GDS thing ie not airline specific.
And I was able to reserve specific seats for John and Exst both :) which on Norwegian long haul is really badly important because there is only one bulkhead seat pair which can lift the armrest: 6D and 6E. Look carefully at the image below from The Points Guy blog and you can see the unmoving armrests for the bulkhead seats and on the bottom right although a bit dark you can clearly see the normal armrest, that's 6D-6E: