Don’t Delete
With the low cost / high storage capacity of current hard drives it makes a lot of sense to keep all of your clients’ masters (and working files) forever.
Fill a drive, put it on the shelf, and pop in another.
In my case I have a four bay SSD dock. One drive holds the current work, another clones this drive, and a third is for daily incremental backups. When the current work drive is filled it just goes on the shelf along with it’s clone, with a note marking the start and end dates of the contents.
As a further backup I clone the full working drive to a huge 24TB USB drive. Redundancy is good. For many years I had everything mirrored in an unlimited storage Google account as well, but the Google enshittifiers pulled the plug on that one.
When I need to find an old project I just look back through emails or billing records to quickly find the date a project was mastered, and thus the right drive, without the hassle of maintaining a formal drive directory listing.
I don’t advertise or charge for this, as I don’t want the potential legal ramifications of running an archiving service, but if anyone asks I just tell them the files will be here until the hard drives fail (which they will eventually, but so far so good), and encourage them to keep their own copies and backups too.
Countless times I have been able to provide replacement files after all manner of tragedies, including stolen computers, hard drive crashes, even house fires.
The goodwill this generates can only be good for business.