As far as we know, the Override: Last City will not come back into rotation before Witch Queen, meaning you would not be able to complete the seal.
However, given that Bungie did make the Shattered Realm activity rotate daily now, there is a possibility that they may do the same with the Override locations to allow for players to catch up, but no guarantees have been made.
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Rip, that’s kinda what I figured. Thanks for the answer! Here’s hoping they change the rotation