Hey!
tl;dr after the next deploy merging two items creates a redirect, merge gadget (and every tool with a similar functionality) needs to be adjusted to the new functionality
There was a change to the functionality of merging items. Whenever you merge two items a redirect will be automatically created. If you merge via the special page nothing changed - except for the item, that is merged into another item now also redirects to that item. The same goes for the API except for using 'ignoreconflicts'- when the item is not left empty after the merge it won't be a redirect and still be there as before without the data that got merged into the other item. (So it will only have the data that would have given a merge conflict.)
Last but not least: The merge gadget will need some adjusting. That's expected since the functionality changed. Please keep that in mind if you have any tool, that merges items.
Cheers,
Lucie