How to add? can demo users have one?

No, demo users cannot have a signature.

I saw demo users from 2006 with signature and avatar, why this is removed?

No idea to be honest. Buy a licence and you will get free avatar and signature option for free :P

But a knife and cut your internet cable. Problem solved.

free = 24 pounds ? nice

thats more like gratis

Some really old demo users got to keep the avatar they had before it was made not available to demo users, I can't recall seeing any with sigs though.