@LACUERDA ORTS, MYRIAM, Thanks for posting in Q&A.
From your description, I know you want to disable InPrivate Browsing on Windows Single App Kiosk mode via Intune policy.
Based on my research, the official document say InPrivateModeAvailability is not supported in Kiosk mode and currently there is no such setting that can disable InPrivate mode in Kiosk mode, because it is designed to protects user data.
If you want to implement this feature in the future, you can submit your feedback to Intune.
https://feedbackportal.microsoft.com/feedback/forum/ef1d6d38-fd1b-ec11-b6e7-0022481f8472
Hope it will help.
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