Does menace require both blockers to survive for the creature to be considered blocked?

Menace requires that a creature can only be blocked by two or more creatures (CR 702.110a). This restriction applies during the declare blockers step when blockers are chosen. Once the creature has been legally blocked by two or more creatures, it is marked as "blocked" and that status does not change even if the blocking creatures are later removed.

The key rule here is CR 509.2: a blocked creature remains blocked even if all creatures blocking it are removed from combat. So if one or both blockers die to a spell cast during the declare blockers step (or at any later point), the originally blocked creature does not become unblocked and will not deal combat damage to the defending player unless it has trample.

This is an important strategic consideration: a defender who double-blocks a menace creature may lose both blockers to tricks or removal, but the attacker's creature will still be blocked and deal no damage to the player (again, absent trample or similar abilities).

Example: You attack with a 4/4 with menace. Your opponent blocks with two 1/1 tokens. You cast a spell that destroys both tokens before combat damage. Your 4/4 is still considered blocked and deals no combat damage to your opponent, even though there are zero creatures blocking it now.

This interaction often surprises newer players, but it is firmly established in CR 509.2 and is consistent with how the blocked/unblocked status works throughout the combat phase.