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Yes it does seem like they could have removed candidates who dropped out. But they load the data in there in a certified process; probably not able to remove Buttigieg and Klobuchar on Sunday.
Ballot design is a whole field of study and the padding helps. @civicdesign has more.
There are usually state laws about how candidates qualify for the ballot...and when they can withdraw. One consideration: might be mail-in ballots, which are sent out early, or not making changes during the early voting days?
