Javier Jacobson wrote:

Anything you add to the cord will stiffen it. This added rigidity will
transmit increased cord bending forces into the device connector causing
the device's connector to fail prematurely.

Exactly. This is what a true strain relief is supposed to do: confine
the stress to the cable rather than transmit it to the connector. Some
cables are reinforced to prevent sharp bends, a common cause of cable
failure, near the ends; but that's not usually referred to as a "strain
relief".