The setting seems to be working as intended even in muli-object editing, but is for some reason missing from any menus. Added to the Snapping panel for the Vertex option.
The AutoMerge Editing option is hard to find, especially to those who are not used to the new topbar. I reckon it would help to have it appear in the Snapping popover as a subsection with its Double Threshold, since this option is mostly used with snapping enabled.

