Yeah I've thought something like this has been needed for a while. I'd been thinking of the idea of adding a log entry at the target as well as the source, and that wouldn't go astray either, but that would probably not be so easy ...

A log entry at the target was separately requested in T118132: Merging pages should add a log entry to the destination page. I personally don't see the point of any of these requests, though - history merging should if done properly be seamless and not need an annotation.

I do history-merging on enwiki (though not as often as previously) and the main reason I don't use Special:MergeHistory (except in unusual cases) is the lack of annotation. I think it should be annotated like any other edit/admin action that affects a page (yes when done cleanly the history should still seem natural, but sometimes history merges hard/impossible to do cleanly).

In T341760#9269957, @Pppery wrote:

A log entry at the target was separately requested in T118132: Merging pages should add a log entry to the destination page. I personally don't see the point of any of these requests, though - history merging should if done properly be seamless and not need an annotation.

In the open source movement and MediaWiki, we tend to publicly log almost everything that takes an action and that isn't sensitive data. Personally, I think it helps with transparency, with anti-vandalism, and with understanding the history of actions taken to a page.