I don't see a sign or anything saying "REMOVING FROM THE FIELD IN THIS MANNER IS TIME'S TERRITORY." I think it's fine.
Agreed. When a theme (Fear) makes sense for a creature to leave the field in a non permanent fashion, then returning to the deck is not only permissible but mandatory design.
However balance is another concern.
Returning 1 creature is denying 1 draw.
Returning 2 creatures is denying 2 draws.
...
Denying a draw is the best form of card advantage in EtG. Card advantage quickly ramps in value. 2 cards of advantage is more than twice as valuable as 1 card of advantage.
Rewind 1 creatures costs 2
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+ 1 card (or 1
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+ 1
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[upgrade] + 1 card)
Rewind 2 creatures would cost more than 6
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+ 1 card (4
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+ 2 cards)
Rewind 3 creatures would cost even more than 10
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+ 1 card
Lets assume the low addition of +1, +2, +3 ... to approximate the increase in value.
Most AoE is balanced under the assumption of an average of ~6 creatures.
Rewind 6 creatures would probably cost at least 37
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+ 1 card.
So mass rewind is way too powerful an effect to ever see in EtG. I think it has to be downgraded from an AoE in order to balance it without making it a "75% of the time this card does nothing" effect*.
*Those effects are bad design. Do not touch them.