At the time of Terri’s mysterious collapse, so-called living wills were only for people who didn’t want to live. For people like Terri, who wanted to receive the normal care which was routinely given by default, there was no reason to put those desires in writing.
The practice of requesting deprivation of routine life saving procedures was new, but not unknown. It was not yet legal to request death by starvation and dehydration, but since that clearly wasn’t her desire, that’s a moot point.
Terri was not dumb for not requesting depravation of care. The overwhelming majority of people, then and now, did not/do not make such a request.
In 1999, it became legal for the first time to kill Florida's disabled by removing their feeding tubes. Any ‘verbal contract’ Michael and Terri might have had before 1990 to kill each other this way, would have been murder at that time and thus illegal.