Singer of Swift Rivers
Flash When this creature enters, put a shield counter on another target creature you control. (If it would be dealt damage or destroyed, remove a shield counter from it instead.) You may cast Merfolk spells as though they had flash.
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Official rulings (6) · Scryfall
If a permanent with a shield counter is dealt unpreventable damage, that damage will be dealt and a shield counter will still be removed.
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.
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.
Removing a shield counter in this way isn't the same as regenerating a creature.
"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.
Shield counters don't prevent players from sacrificing creatures.