A Shield spell is cast as a reaction to an attack that hits. It applies its AC bonus even against the attack to which it is a reaction, meaning that it can make the attack that hit, retroactively miss and thereby not do damage.
Shield ‘interrupts’/potentially cancels its trigger, as stated in the DMG in the ‘Adjudicating Reaction Timing’ section.
Since the hit has retroactively become a miss, what happens to effects that were triggered by the hit in the first place? Are they undone as well, or do they persist?
This question has two answers of nearly equal popularity, with one arguing that other effects triggered by the hit still persist, and the other arguing that they do not. On the side of the argument that they do not occur is a (now-unofficial) tweet from Jeremy Crawford stating
If the attack has a special effect that relies on it hitting, that effect doesn’t occur if the attack is turned into a miss.
However, the discussion therein focusses on a narrow interpretation of ‘effects’ as discrete, measurable things that happen to specified targets – the hit on the caster of the Armor, and the resulting damage to the creature making the attack.
Are the ‘special effects’ of an attack referred to limited to things like damage or conditions that affect just the target of the attack, or do they include intangibles like knowledge for all intelligent observers?
A party is traversing wilderness with abundant natural cover as well as opponents who are seeking to ambush them.
A hidden opponent fires a missile weapon at a party spellcaster and hits. In response, the spellcaster uses the shield spell and turns the attack into a miss. However, the ambusher has now revealed their location to the spellcaster and everyone else in the party, since according to the rules of Unseen Attackers and Targets
If you are hidden–both unseen and unheard–when you make an attack, you give away your location when the attack hits or misses.
The party members begin to defend themselves and target the ambusher.
The next round, a second hidden ambusher also fires at the spellcaster, hits, and also reveals their presence to the caster and the rest of the party. The spellcaster again uses shield, and again converts the hit into a miss.
This second ambusher, however, possesses the Skulker Feat (PHB 170)
When you are hidden from a creature and miss it with a ranged weapon attack, making the attack doesn’t reveal your position.
Since the attack was now actually a miss, does the knowledge of ambusher’s location persist as revealed, or is this knowledge undone?
…the shield spell travel back in time, converting the hit to a miss before the party had knowledge of the second ambusher’s location?
…knowledge of the ambusher’s location count as a ‘special effect’ of the attack hitting, that now does not occur since the attack resolved as a miss?
…the ‘special effect’ of the knowledge of the ambusher’s location get removed from only the caster, while their companions still know where the ambusher is?
…the knowledge of the ambusher’s location not count as a ‘special effect’ of the attack, but rather as mundane effect, that is not changed by the shield spell?
A good answer may use the Crawford quote or not. If it does, it should convincingly interpret what is a ‘special effect’ and to whom it applies. If the answer does not use the Crawford tweet, it should explain from where it draws its reasoning.