Even if the feat is not implemented, we already know that Attacks of opportunity and the shield are because that was in the gameplays. We also know that you automatically use them the first time you can when it´s possible because we all saw Lae´zel doing it.
One thing about reactions is that in the TT D&D5e game you can choose to take your reaction or not, or to whom you want to react. That´s logical because you only have one per turn.
Maybe you want to use your shield against the ogre´s greatclub bash, not against the goblin´s knife stab. Perhaps you want to use your AoO against the mage that flees, not against the armored knight. Maybe you do not even want to spend your spell slot in it this turn.
That would not be possible if you have automatic reactions.
And I´m pretty sure any rogue would have uncanny dodge, warriors would have the fighting style "protection", lore bards would have "cutting words" and battlemasters like Lae´zel have the battlemaster´s parry. All of them and more were discussed in another thread. We stopped counting at 15 different abilities, feats and spells that require reactions, all of them of the PHB so they possibly be in the game.
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