Hi sammentions
have a look at these new changes: https://techcommunity.microsoft.com/t5/intune-customer-success/new-contact-sync-scenario-available-with-outlook-for-ios-on/ba-p/1063632
You can enable the contact sync via the e-mail device configuration profile. That way, contacts are treated as "company documents". Hence, they cannot be accessed by any non-managed App (e.g. WhatsApp, Twitter, etc.).
Additionally, you need to create an App Configuration Profile (as described in the link above) to block the contact sync that can be done via the Outlook app. All contacts synced via the Outlook app are not treated as company documents.
If you do not want to create an App Configuration Profile, you can use the normal App Protection Policy and disable the Contact Sync there: http://iOS App Protection Policy (the setting you want is: Sync app with native contacts app set to Block). However, this applies to all targeted apps. To only block contact sync for Outlook you need an App Configuration Profile.
I hope this helps you. Please provide us with feedback, whether this solution meets your requirement.
Best regards,
Labinot
I hope this helps.