The real reason for only loading 19 rounds in a 20 round magazine or 29 rounds in a 30 round magazine is quite different than the old wives tale you reference.
The actual reason relates to the difficulty of inserting a fully loaded magazine into the firearm deep enough until the retaining latch engages when the bolt carrier group when in the closed position. The top round in the magazine must push the other cartridges, resisted by the spring force, down until the latch engages. If not properly secured, the magazine will drop from the firearm, possibly at a very inopportune time.
The magazines are easily inserted and retained when the bolt carrier group is locked in its rearward position. Now you know the truth but that old urban legend will probably continue to be retold and retold. P.S. I have many G.I. 20-round M16 magazines that have been fully loaded since 1972 and they function perfectly. There is and was nothing wrong with the original spring design or materials from which they were fabricated.
Thanks for that. Always good to see a false myth take one on the chin.
Best,
L