merged:yes doesn't seen to work. Nor does
How would I search for the merged questions?
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The most right way would be to go to Annotated posts > locked > merged. Though this search is not very flexible, as you can't specify another parameters.
The most close alternative using advanced search is
duplicate:yes locked:yes answers:0, however on MSE the advanced search option does give two results less than the annotated posts; and on SO, otherwise, excess 302 results.