7 Jun 2024
Players can respond to a spell or ability whose effect includes proliferating. Once that spell or ability starts to resolve, however, and its controller chooses which permanents and players will get new counters, it's too late for anyone to respond.
7 Jun 2024
A creature with a shield counter on it may still be destroyed by state-based actions if it has damage marked on it equal to its toughness or has been dealt unpreventable damage by a source with deathtouch.
7 Jun 2024
You don't have to choose every permanent or player that has a counter—only the ones you want to add counters to. Since "any number" includes zero, you don't have to choose any permanents at all, and you don't have to choose any players at all.
7 Jun 2024
"Shield" is not an ability that creatures have and shield counters are not keyword counters. If a creature with a shield counter loses its abilities, the shield counter will still protect it as normal.
7 Jun 2024
When you proliferate, you can choose any permanent that has a counter, including ones controlled by opponents. You can choose any player who has a counter, including opponents. You can't choose cards in any zone other than the battlefield, even if they have counters on them.
7 Jun 2024
If a permanent with a shield counter is dealt unpreventable damage, that damage will be dealt and a shield counter will still be removed.
7 Jun 2024
Shield counters don't prevent players from sacrificing creatures.
7 Jun 2024
If a permanent that would be dealt damage has more than one shield counter on it, that damage is prevented and only one shield counter is removed.
7 Jun 2024
If a player or permanent has more than one kind of counter on it, and you choose for it to get additional counters, it must get one of each kind of counter it already has. You can't have it get just one kind of counter it already has and not the others.
7 Jun 2024
Removing a shield counter when a permanent would be dealt damage or destroyed isn't the same as regenerating that permanent.