Block slots have been removed and are no longer available.
Block slots let you place multiple ArrivesBy blocks on a single product page, each showing a different delivery estimate. This feature has been retired — the Block slot setting no longer appears in the theme editor, and rules can no longer be scoped to a slot.
If you want to show different delivery estimates for different shipping options on the same page, this capability is planned as part of an upcoming Widgets feature. We don't have a release date to share yet.
If you have questions about your current delivery-estimate rules, see How rules work.