Cats without homes aren't necessarily euthanised now, even without chips.
For feral colonies, there are trap and release programs that spay/neuter cats and nick their ears before letting them go. Chipping them at the same time shouldn't be a problem.
Single feral cats are currently chipped and placed with people who have barns or outbuildings.
I'm sure that some cats do have to be euthanised, but I certainly don't think that's anyone's aim.