Prevent Macbook pro WIFI from being deleted when I remove from iPhone;

I have done this in the past. Unfortunately, it made no difference. Also, I believe that reinstalling the entire OS recreates those preference files.

However, if the iCloud Keychain still had those networks listed,,,, those preference might be repopulated with the old networks. It could depend on what order all of this is done. Maybe:

1st Delete iCloud Keychain

2nd Remove those preference files

I think this would have to be done on all of your devices that are using the same iCloud ID or it would pull those networks again and repopulate them?

This page has some insight into WiFi issues

Selectively disable iCloud keychain syncing for WiFi passwords - Ask Different

Thanks for letting us know you've tried those steps listed above. Try this instead. Open Messages and click Messages > Preferences, look for the option to "Save history when conversations are closed" and make sure that this is unchecked.

This exact issue has also been a thorn in my side, as I frequently do not want my iPhone on the office Wi-Fi network for one reason or another.

A loophole that has been working for me lately (your milage may vary) is to forget the network in question from the iPhone, and then rejoin on the Mac. Its probably a bug, but I've found that I get several hours with the iPhone on cellular and my computer still on Wi-Fi this way. The phone does, unfortunately, rejoin the network once iCloud Keychain figures out what is going on and syncs up.

The only permanent solution I have found so far is to disable iCloud Keychain completely on the iPhone. That way, since the iPhone will not share passwords with your other devices, when you forget the Wi-Fi network on the iPhone the Mac will be unaffected.

Since I quite like iCloud Keychain and use it frequently, I find this a less than satisfactory answer. However, it will result in the desired outcome here.