Yes, encrypted memos are exclusive to shielded outputs, so the transaction must have at least one shielded output (one shielded recipient address). (So no wallet will support attaching an encrypted memo to a transaction with purely transparent recipients, because it’s not possible.)
The staked poll held last year had an interesting design: a transaction containing both a t2t nonzero transfer (to record the staked amount) and a t2z zero-ZEC transfer (just to record the vote via a memo).