Legality in The US
The clause in the National Voter Registration Act of 1993 (NVRA) which has been interpreted to prohibit voter caging is:
Pursuant to the NVRA, a voter may not be removed from the voters list unless (1) the voter has requested removal; (2) state law requires removal by reason of criminal conviction or mental capacity; (3) the voter has confirmed in writing that he has moved outside the jurisdiction maintaining the specific voter list, or (4) the voter both (a) has failed to respond to a cancellation notice issued pursuant to the NVRA and (b) has not voted or appeared to vote in the two federal general elections following the date of notice.
Under this clause voter caging may be legal if the primary purpose is to identify those who are not properly registered to vote and prevent them from voting illegally, but not if the purpose is to disenfranchise legitimately registered voters on the basis of a technicality.
Read more about this topic: Caging (voter Suppression)