How to block synchronization of certain file types in OneDrive Sync

1.Click on Microsoft 365 App Launder > Admin

2.Under Admin centers, choose SharePoint Admin Center

3.Click Settings, then Sync setting next to OneDrive

4.Check the box next to Block upload of specific files types

5.Type in the file extension(s) for the file types you want to block from syncing. One file type per row. In my case, I just want to block the dwg (AutoCAD) file type. Click Save

6.You might need to wait up until 8 hours for this to take effect

Experience for the end-users

1.Once this takes effect, users will start seeing warning messages and notifications next to the OneDrive Sync Cloud icon. It might say something like: Item name is not allowed in cloud.

2.And, of course, they will see a detailed log of error messages once they click on the OneDrive Blue Icon.

3.Once users open their OneDrive and Libraries in Windows Explorer, they will see the detailed Status of which files synchronized and which ones did not.

Credit : https://sharepointmaven.com