I'v been told that walking with a heavy foot-fall will deter snakes. Is this really true?
As with any wild animals, different species and different individuals are going to respond differently in different situations.
Some snakes are extra-ordinarily docile, and no amount of heavy-stomping will disturb them, unless you stomp on them.
So, to answer the original question, walking with heavy footfall is neither foolproof, nor sustainable for long hikes: resulting in very sore feet at best, and stress-fractures at worst.
Best is to keep an eye on your surroundings, and be careful not to place your foot somewhere you can't see what might be lurking around it - ie, stepping over a log, walking beside an overhanging rock, in deep grass, etc.
Stepping on a snake is an obvious no-no, but even passing within striking distance can result in an attempt depending on the disposition of the particular animal.
I hike in Arizona, which is super snakey!!! An easy deterrent is to hike with wooden sticks (I make mine from yucca or agave stems) and tap in front of you as you walk. 99.9% of snakes will get out of the way or let you know they are there (rattlesnakes) long before you reach them due to their incredible ability to sense vibration. I have been doing this for 10+ plus years and that along with some common sense like tapping logs and large rocks prior to stepping over them etc. has served me well! Snakes are beautiful critters!