Does deathtouch from a source deal lethal damage through a damage prevention effect?

Deathtouch works by making any nonzero amount of damage dealt by the source sufficient to destroy the creature it damages (CR 702.2b). The key word is dealt — deathtouch only matters after damage actually resolves and is applied to the creature.

Damage prevention effects are applied in the damage-prevention step (CR 615.3), which occurs before damage is considered dealt. If a prevention effect reduces incoming damage to zero, no damage is ever dealt, so deathtouch has nothing to act on. The creature survives untouched.

If a prevention effect only partially reduces the damage — say, a creature has a shield that prevents 2 damage and a deathtouch source deals 3 — then 1 damage is still dealt. Because deathtouch says any amount of damage is lethal, that remaining 1 damage is still lethal (CR 702.2b, 510.1d).

Concrete example: Your opponent attacks with a 1/1 with deathtouch. You block with a 4/4 that has a regeneration-style effect preventing the first 1 damage it would take each turn. All 1 damage is prevented, so 0 damage is dealt. Your 4/4 does not get a deathtouch counter and is not destroyed.

So the outcome depends on whether prevention removes all the damage (creature lives) or only some of it (deathtouch still applies to whatever damage gets through).