The bug
If a player blocks a thrown trident, it will grant the "not today, thank you" advancement. The description of "not today, thank you" is "Deflect an arrow with a shield", the id is story/deflect_arrow and the translation keys use the id.
Linked issues
duplicates 1
Comments 8
Looking at the advancement file, it just says it needs to block a projectile's damage.
I'll double check later today in game which projectiles specifically it affects.
Simply changing the description to "Deflect a projectile with a shield" won't help here, since this implies Wither_Skulls, Fire_Balls, Eggs, Snowballs, etc.