Uncage the Menagerie (RotGP 3.1) has deal 20 or more damage to creatures as one of the objectives. I had my Ruin Rat kill two opposing creatures while having 20+ attack, and still failed the objective because deathtouch reduced the damage to equal the opposing creature’s toughness.
Edit: Strangely, lifelink will heal me for the correct amount despite changing nothing else about the above circumstances.
I think they just set it so deathtouch would deal lethal damage regardless of what the creature with deathtouch’s attack actually is. It just never mattered before they added creature damage objectives. It definitely isn’t dealing one damage, as Uncage only cast 4 creatures, and the objective ended up counting as something in the teens.