I wonder when cars become inoperable without using a smartphone.

People had paper maps and often precise directions that they had on their laps or a human navigator using those maps or directions. They stopped at gas stations to ask for directions (or didn't--guys refusing to stop to ask for directions was something of a trope). So obviously they managed but probably in ways a lot of people today wouldn't find satisfactory.

How did you manage before smartphones and even email/text for meeting people and generally coordinating activities generally? People managed.

That is a different question. I can use my car without a smartphone. However if I buy a new car will I be able to use it if I break my smartphone?

Sure. You may just not have a navigation system or be able to park in some places. I don't need a smartphone to drive in general and don't always plug it in locally. (I may more or less need a transponder on some roads or have a bit of hassle.)

So far. Will that continue is the question.

At a minimum, connectivity would have to be much more universal than today. Certainly, cellular is very far from pervasive and even satellite isn't 100%.

The smartphone could bridge gaps in connectivity with buffered data, depending on the purpose.