When attacking a player who is blocking using a shield on a vanilla 1.16.3 server, the player hurt sound gets played on the attacker's client even though that player is not taking any damage. The player hurt sound is not played on the blocker's client.

When disabling their shield with an axe, the "item breaks" sound is played on the blocker's client, but not on the attacker's client, so you can't tell whether or not you successfully broke their shield.

video of bug