Feature summary:
T363906 introduced a new checkbox to abuse filters, to protect PII-sensitive variables. The checkbox is labeled "I understand that details of this filter will be hidden from users who cannot see protected variables. This action is permanent and cannot be undone."
T364485 introduced a warning when trying to save an unprotected filter with PPI-sensitive variables.
I think a similar warning might be useful when changing an existing filter from its current status to protected: Even though the checkbox already says, this action cannot be undone, one might not fully realise what they are doing and accidentally (e.g. while trying to test the new checkbox) change an existing filter which doesn't need to be protected.
I suggests adding a warning asking to confirm the change when clicking "save" and changing an existing filter to protected – at least while the feature is still new, the warning might be useless once everyone has learned about protected filters.
Use cases / benefits:
Prevent admins from accidentally changing existing filters to protected in case the current checkbox helper text isn't taken seriously and the filter doesn't need to be protected.