That's correct. It's not just limited to combat damage.
702.2c. A creature with toughness greater than 0 that’s been dealt damage by a source with deathtouch since the last time state-based actions were checked is destroyed as a state-based action. See rule 704.
Yes, there is no time when thundermaw will be on the field and not have deathtouch, tho more importantly it will have deathtouch when its etb ability resolves.
let's say Thundermaw Hellkite has deathtouch does it's 1 damage to all flying creatures my opponent controls will be lethal?
thanks
702.2c. A creature with toughness greater than 0 that’s been dealt damage by a source with deathtouch since the last time state-based actions were checked is destroyed as a state-based action. See rule 704.