Best document shred/free space shred program for Imac Tahoe?

Shredding won't work on SSDs because once you delete the file, you can no longer access the block where the "deleted" data was stored. The old block which contained the "deleted" data will be processed by the SSD's controller at some point to reset that block (aka erase it) so that block can be re-used.

Also, with the APFS file system, you cannot overwrite a file because the active of overwriting a file causes the new data to be written to an entirely new physical location within the file system & the link to the old copy removed.

If you are concerned about security, then enable Filevault so that the data on the boot drive is encrypted. With the 2018+ Macs, the data is always hardware encrypted, but enabling Filevault adds an extra layer of protection to the encryption keys.

Protect data on your Mac with FileVault - Apple Support

Now if you are using an external Hard Drive as opposed to an SSD to store this data, then you could utilize a shredding app or free space cleaner if you are using a file system other than APFS. However, if you are that concerned about security for those items, then encrypting the external Hard Drive should be done, but these days macOS will only allow users to encrypt an external drive with the APFS file system where shredders & free space erasers won't work. IIRC, macOS will no longer allow a user to create an encrypted MacOS Extended volume. So we are back where we started.

Encrypting your drive is the best option, just make sure to use a good password and keep the recovery key handy & safe.

NickapTopeka wrote:

The Intego software (NetBarrier, Virus Barrier and Washing Machine) will work with Tahoe.

Besides not needing any shredding apps there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This documents describe what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community and Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

Unless you're using a true VPN tunnel, such as between you and your employer's, school's or bank's servers, they provide false security from a privacy standpoint.  Read these two articles: Public VPN's are anything but private and Security Risks: The Dangers of Using Free VPNs (eccu.edu).  

Additionally, a new study ("Apple Offers Apps With Ties to Chinese Military”) is specifically about VPN apps in Apple’s App Store.

So uninstall the Intego software according to the developer's instructions and make sure all supporting files are removed also.

It appears you have been misled and are laboring under a number of misconceptions. Your most recent reply introduced several more such misconceptions, and strongly suggest your computing practices are in dire need of review.

HWTech provided accurate guidance that really can't be improved upon.