When Apple shipped macOS 10.15 Catalina, one of the new features were Siri Suggestions in various apps. And since people might get used to it, some other – me included – would like to disable it. This is why I searched for a solution to disable Siri Suggestions. For me personally, I got most annoyed by the suggestions in the favorites page of Safari but it is also possible to have these new suggestions displayed in other apps like Reminders, FaceTime, Calendar, Contacts, Mail, Messages and Podcasts.
At least these are the builtin apps from Apple that have native support for Siri Suggestions. Even if you have disabled Siri on your Mac, the suggestions would be enabled and displayed by default in all these apps. And maybe in more …
To disable them, you have to go to the System Preferences, then to Siri and there you can find a button called “Siri Suggestions & Privacy …” in the lower right corner. You will see, even if you’ve disabled Siri, this button is not disabled and can be used. By clicking on it you will see a long list of apps where Siri Suggestions can be disabled and even some additional apps where Siri is allowed to learn from to create better suggestions.

Now just go through each app and disable the Siri Suggestions by de-selecting the checkbox of “Show Siri Suggestions in App”. If you want to, you can also directly disable Siri to learn from this app by using the appropriate checkbox (“Learn from this App”).
You don’t need to restart, the changes will take place immediately and disable the Siri Suggestions.