Polls without an end date are much more common than I thought, apparently this is a default in Misskey. So here is an update of FEP-9967: Polls: https://codeberg.org/fediverse/fep/pulls/706
Now endTime is optional:
Question objects MAY have an endTime property indicating the time when the poll ends.
I also added a sequence diagram. What's the best way of describing activities in such diagrams?