
Now there would still be plenty of use for treecat-sign but keyboards coupled with a voice synthesiser (in 2,000 years they've surely developed something better than what Stephen Hawking uses) would be pretty useful for cats to be able to use. Another possibility is a portable device either using buttons like old SMS texting or a small touchscreen etc. Something lightweight that treecats can carry with them.
The flip side is that treecat sign only being understandable by a small segment of the population may be useful to obfuscate communication so outsiders can't understand what is being said.