I'm wondering if the correct wording would be "~ has indestructible unless it has been dealt damage this turn."
I'm also curious if it would be possible to define lethal damage.
Searching on gatherer Ogre Enforcer is the only creature to refer to lethal damage, but it means there is a precedent for it.
"~ has indestructible as long as lethal damage isn't marked on it."
"~ has indestructible unless lethal damage is marked on it."