Might be a third party plugin or extension ...
Troubleshoot Safari plugins and extensions:
From your Safari menu bar click Safari > Preferences then select the Security tab.
Deselect: Allow all other plug-ins. Quit and relaunch Safari to test.
If that made a difference, instructions for troubleshooting plugins here.
If it's not a plug-in issue, select the Extensions tab. If there are any extensions installed, turn that OFF, quit and relaunch Safari to test. If that made a difference, uninstall one at a time, quit and relaunch Safari until you find the culprit.
And check Safari > Preferences > Advanced. Accessibility eanbled ??
The Mac showing the unwanted behavior is a very vanilla system with no client extensions and no client plug-ins. I turned plug-ins off anyway and saw no change in behavior. I had previously tried the Zoom Text Only option and it had no effect that I could see. A friend reports the behavior is site specific: CNN and the NYTimes zoom the width but Wikipedia reflows the zoomed text. From that observation I'm expecting that there is a css default behavior that changed somewhere, and if I can figure out which magic parameter changed I can put it back. It may also be an issue on the server side (also on this machine) that I can adjust to model the behavior of the wikipedia site. I shall keep looking. Thanks to you and Mr. Davis for your responses.
I have discovered that if you have Zoom Text Only turned on, and use Command/+ and Command/- for zooming, you get the desired behavior. If, however, you use the trackpad to pinch zoom in/out, you get the unwanted behavior. I had been using the latter form exclusively. I was not aware of that semantic difference between the two methods of zooming in Safari. I'm still going to look into how WikiPedia manages zooming.