I heard some people mention that files are deleted from flash drive for good, there is no tracing back. Is that really true?
Yes and no; it depends on your definition of “gone for good”.
What they were likely talking about was that deleting a file from a flash-drive is always equivalent to holding ⇧ Shift when deleting the file.
This is because Windows only puts a Recycle Bin on fixed volumes like hard-drives, not removable volumes like flash-drives, memory cards, floppy disks, packet-writing CD+RWs, network drives, or
substituted drive letters (even ones mapped to fixed-disks). Therefore, if you delete a file in Windows from a hard-drive with just Del instead of ⇧ Shift+Del, you should be able to restore it from the Recycle Bin, but deleting a file from a removable media with Del is the same as using ⇧ Shift+Del to permanently delete it since it has no Recycle Bin.
However, for performance reasons, permanently deleting a file doesn’t actually delete the file. Instead, the system only marks it as deleted and its disk-space as free for use by new files. Until its clusters are overwritten by other files, it can theoretically be recovered, and if there is a lot of free space, then it could take a while before it gets overwritten—of course if you are actually trying to recover an accidentally deleted file, it is likely to immediately overwritten even with plenty of free space
If not, how do I permanently delete files from it? I have a sensitive file on a flash drive and before passing the flash drive around, I want to make sure nobody will be able to see that file.
Use a secure-deletion tool. They will overwrite the file before deleting it. The better ones also delete the directory entry to wipe out even its filename and the best will even overwrite its meta-data like its size and timestamp. Some can also wipe existing free space which can be handy to wipe previously deleted files, however it can take a while if there is a lot of free space since it is writing to every byte of that space, e.g., wiping a drive with 300GB of free space is like creating a 300GB file. Most provide one or more techniques like different patterns of bytes to overwrite with and number of repetitions.