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.

“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.

If a permanent with a shield counter is dealt unpreventable damage, that damage will be dealt and a shield counter will still be removed.

Shield counters don't prevent players from sacrificing creatures.

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.

If the attacking creature loses its last shield counter at the same time it deals combat damage to a player (for example, if the attacking creature has trample and would be dealt damage by a blocker), then you will not be able to pay the cost of removing a counter when the triggered ability resolves.

Neither the triggered ability that puts a shield counter on a target attacking creature nor the triggered ability that removes a shield counter from it are optional.

Removing a shield counter in this way isn't the same as regenerating a creature.