Hi @IceQueen
Thank you for posting.
Due to the software update, there is no Advance Condition now, You could use expression on both sides of Condition to achieve your needs.
I have tested it on my side and it can delete the file that 14 days after modified in the folder. You can change 14 to 90.
Please refer to as below:

The Expression on the left of Condition:formatDateTime(item()?['Modified'],'yyyy-MM-dd')
The Expression on the left of Condition:formatDateTime(addDays(utcNow(),-14),'yyyy-MM-dd')

After Flow runs. The file ‘master student list’ will be deleted.


Hope the content above may help you.
Best Regards
Lily