Myth, mostly. Salt SLOWS yeast, it doesn't kill it - and that slowdown is a feature. A controlled, slower fermentation is where bread develops actual flavour. Saltless bread tastes flat and ferments recklessly.
The kernel of truth behind the myth: salt in direct prolonged contact with yeast (like a heap sitting on the yeast in a bowl overnight) can damage it. The practical habit that makes it a non-issue - whisk the salt into the dry ingredients rather than piling it directly on the yeast, then proceed. Everything in one bowl is fine once distributed.
The rule that actually matters: never DOUBLE the salt. Correct amount, mixed through, and salt is bread's best friend, not its assassin.
