What happened:
I ran the following command "/scoreboard objectives add Blocked minecraft.used:minecraft.shield" and the scoreboard objective was created successfully. Then I ran the second command "/scoreboard objectives setdisplay sidebar Blocked" so that I could see when my score increases.
Then I tried to block all kinds of things (including skeleton arrows, blaze fireballs and zombie melee attacks) with shield in my offhand slot and the scoreboard score didn't increase.
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However, this issue is a Duplicate of MC-87949.
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