The sync client prompt mass delete notification is a new Microsoft 365 feature.
Whenever someone deletes more than 200 synced OneDrive or Team Site files at once from their device, the OneDrive sync client will prompt the user to confirm it was intentional. If it was accidental, the user can tap “Restore Files” to bring them back, or if the action was intentional, they can choose to tap “Remove” which will complete their action. If a user wants to bypass this new feature, they can select the “Always remove files” box before tapping “Remove” to skip the prompt for future mass deletions.
The roll out will be completed worldwide by the end of February 2019.