Does wither damage count as damage for the purposes of deathtouch?

Wither and deathtouch are both replacement/additional effects that modify what happens when a creature deals damage, but neither ability cancels or overrides the other. When a creature has both wither and deathtouch, all of that creature's damage is dealt as -1/-1 counters (via wither) AND is considered lethal for deathtouch purposes (via deathtouch).

Under CR 702.90 (deathtouch), any amount of damage dealt by a source with deathtouch is considered lethal damage. Under CR 702.79 (wither), damage dealt by a source with wither is dealt in the form of -1/-1 counters instead of the normal damage-marking system. Crucially, CR 119.3 clarifies that placing -1/-1 counters via wither is still "damage" for all game purposes, including deathtouch.

This means a creature damaged by a wither+deathtouch source is destroyed by the state-based action check (CR 704.5g), because any amount of wither damage still counts as lethal deathtouch damage. Neither ability requires the other to be absent.

Concrete example: Your Kulrath Knight (wither) somehow also gains deathtouch. It deals 1 damage to your opponent's 5/5. That 5/5 receives one -1/-1 counter (wither) AND the damage is considered lethal (deathtouch), so the 5/5 is destroyed by state-based actions even though its toughness hasn't dropped to zero from counters alone.

The two abilities are completely compatible and stack. A creature with both wither and deathtouch is especially dangerous because it both permanently weakens survivors and kills anything it touches.