How to securely `shred` a file in Mojave/APFS?
The answer Apple implies:

You don't have to worry about that, since each file's data is not stored contiguously in the drive, but spread out across several physical and logical blocks. Removing a file from the file table makes it nearly impossible to read with forensic software/hardware.

Sources: About Apple File System | Apple File System Reference

The answer that may or may not work:

brew install coreutils
shred Secrets.txt


The documentation for shred doesn't explicitly call out APFS, but from what it says, it might be the case that the data is not necessarily overwritten.

If you wish to engage in determining this, there are contacts and other related links here: http://www.gnu.org/software/coreutils/