Settings -> Spotlight -> Turn off everything you don't want. The first few times you open it it even tells you this.

Admittedly I haven't confirmed that this results in no API calls in the latest few macOS releases but it definitely used to stop them, at least a few years ago.

you realize this is very laborious. every time you install a new app - or sometimes when you update an app - you have to do it again.

also, search for "improve" in settings. I'll bet you didn't find "Help Apple Improve Search" in the results. Ironic on many levels.