Postgresql - how to securely delete the storage freed when a table is dropped?
drop table removes the table files from disk filesystem. 

If you need them securely erased you could perhaps look for a filesystem that supports that.

On linux SSD filesystems, mounting with -O discard (or running fstrim after dropping) will hide the content of the deleted files from consumer-grade forensic tools (like photorec, dd, etc.) however nation-states with electron beam probes and the like may still be able to access the data if they can gain physical access to the media.

On magnetic disk running sfill will shred the deallocated blocks.