Chain "stretch" is something of a misnomer. The metal of the chain's link plates does not elongate permanently nor do the link-pins bow because, although the plates are highly stressed, its not enough to cause plastic elongation.
Temporary elastic stretching of the upper run of chain does occur, but it is low under normal pedalling loads, and can be ignored. Under maximum pedalling loads using a small granny-ring, it is not insignificant being up to about 0.25% ; but it falls off rapidly after much of the load is transferred to the first few teeth of the granny-ring, and for more normal pedalling loads.
The "stretch" of real concern occurs where pins contact and rotate against swaged bushings within the chain joints ; they wear here causing play. When the chain is under tension, this slack in each joint accumulates, and the chain's effective length increases permanently - this is what is termed "stretch" - but it is actually wear.