Edit: it has since appeared, but I don't know how I did it!
I have created a view of revisions of a custom entity type. I would like to group them by entity ID. But I don't see the entity ID available as a field, only the revision ID. If I make a view of content revisions, the Content ID is there. But it's missing for custom entities. I also tried creating a relationship to the entity but that option is missing too. Any ideas how to group by entity ID?