Thank you kindly for trying that, and also for sharing what happened!
Re: I tried it but then found out it had to do with an 'Intelligence & Siri ChatGTP extension' setting.
In iOS 18.3 : Seems we have the choice to keep Apple Intelligence on by default, or to turn it off. (Apple support has videos available on YouTube
if you'd like to know more about it: Eg: paste "Apple support + Writing Tools with Apple Intelligence" in the search bar at the top of a YouTube webpage.)
After you turned off predictive text: Seems Siri is offering the ChatGP extension (to use with writing), which you can choose to set-up or sign out of. That's new! Am as surprised as you :-)
Apple Support's article below may help explain more about it
Use ChatGPT with Apple Intelligence on iPhone - Apple Support
Eg: Describes the various features you can choose:
Also says how to: "Turn ChatGPT off
- Go to Settings
, then tap Apple Intelligence & Siri. - Tap ChatGPT, then turn the ChatGPT extension off.
If you want to prevent Siri from suggesting ChatGPT when you make a request, turn off ChatGPT, then turn off Setup Prompts.
or
To block access to ChatGPT, see Block access to ChatGPT."
Eg says:
- Go to Settings
, then tap Screen Time. - Tap Content & Privacy Restrictions, then turn on Content & Privacy Restrictions.
- Tap Intelligence & Siri, tap ChatGPT Extension, then tap Allow or Don’t Allow."
For finding out about About Apple Intelligence, and all of the features:
See Get started with Apple Intelligence on iPhone - Apple Support
or
If you want to turn the whole of Apple Intelligence off:
You can go to Settings > Apple Intelligence & Siri, then tap the button next to Apple Intelligence.