Being invulnerable means damage is prevented.
If people were going by the opposite of the effect, then that would mean that anti-damage is prevented. Since healing is “anti-damage”, then the “opposite” of Uther’s shield on the enemy team would be healing prevention rather than damage prevention.
However, another way of having the same effect (since ana prevents healing on a basic ability compared to the uther heroic) would be to turn healing they get into damage they take afterward. So then the healing is prevented, and weaponized.
However, since the inquiry for the topic is "what if healing dealt damage (ie, anti-damage became damage) the fixation on other beneficial effects having an “absolute extreme opposite” was not offered, and it is rather ridiculous.
Since the uther shield does not heal, then the offered status is exaggeration esp since the state of one being temporary in one duration does not match the effect of the other as proposed. So the “instant annihilation” is further demonstrated to not be the ‘opposite’ of the indicated effect unless it then “revived” the target after the few second duration of the shield.