User Configuration > Administrative Templates > Microsoft Excel 2016 > Excel Options > Security > Trust Center > Protected View
User
HKCU\software\policies\microsoft\office\16.0\excel\security\protectedview
This policy setting controls whether text-based files (.csv, .dif, and .sylk) opened from an untrusted location are always opened in Protected View. If you enable this policy setting, text-based files opened from an untrusted location are always opened in Protected View. Users will not be able to change this setting under File > Options > Trust Center > Trust Center Settings > Protected View. If you disable or don’t configure this policy setting, text-based files opened from an untrusted location are not opened in Protected View, unless users have changed this setting in the Trust Center. Note: This policy setting only applies to subscription versions of Office, such as Microsoft 365 Apps for enterprise.