> Confusingly, sometimes Amazon pages for out-of-stock items include a button to notify when something is back in stock, but most don't. They just say "we don't know if or when this will be in stock."

I think that’s a difference between a seller “owning” the listing for an FBA SKU such that Amazon can see in their intake logistics that more of that item is coming to their warehouse; vs. a non-FBA SKU where they don’t have that visibility; or vs. an item that has gone defunct where no seller is claiming responsibility for restocking the SKU any more.