Required at NRA matches. Watch TV coverage and you'll see them. Some ranges require them all the time as well.
From NRA Conventional Pistol Rule book.
3.21 Empty Chamber Indicator (ECI) - An ECI is required in all NRA Sanctioned Pistol competitions to
indicate the slide is open and the chamber is empty. The ECI, when inserted into the revolver or pistol must extend
into the empty chamber or cylinder and must be externally visible.
Same rule in the High Powered Rifle (Camp Perry) rules.
Regardless that is says ECI is
"required in all NRA Sanctioned Pistol competitions" this rule is NOT listed in the Action Pistol rule book. I must be OCD because this type of inconsistency drives me up the wall.