You could put your onion address into an “oh by code”[1] and just write it down … or chalk it on the sidewalk for someone to see … post it on a physical bulletin board.. hold it up on a sign…
This way you could establish communication with an unknown future party, totally offline.
[1] https://0x.co
Trying to repurpose hex literal notation as a "recognizable" URL shortener seems like a questionable idea. At least write it as 0x.co/FFFF so it's obvious to readers how to interpret it.
If you're printing something why not go with a QR code?
If you can use a QR code you probably should.
However, if you're walking down the street and need to quickly generate and apply a message, how will you pass along a QR code to an unknown future viewer ?
Can you draw a QR code with chalk or freehand with a pen, etc. ?
I will admit that the use-cases for "oh by codes" are weird and infrequent but I am convinced they will emerge ...
I don't disagree that URL shortening is incredibly useful at times. Merely that writing out the whole url is almost certainly a better approach and that any sufficiently short domain name is fit for purpose.