I disagree with your point;
Azure Backup vault are not relevant
That's precisely the solution here that allows you to keep a backup of your data whilst deleting the storage account.
After backing up the Files from the storage account to a backup vault it will create a backup policy and a delete lock on the storage account.
You'll need to stop the backup policy, choosing to retain the data, and then go to the storage account and remove the AzureBackupProtectionLock before then being able to delete the storage account itself. (Note this applied to "Files" only in the storage account, not "Blobs".)
I'd suggest creating a new storage account and becoming happy with this process (and that of restoring to a new account) if you're not familiar with it to avoid losing any data.
Where you have a very defined objective of wanting to delete data, but setting out guardrails for the teams using the storage accounts a safer method could be Lifecycle management policies. You would defined a number of days to either move files to a cheaper, less accessible tier or deletion. https://learn.microsoft.com/en-us/azure/storage/blobs/lifecycle-management-policy-configure?tabs=azure-portal