It would be a good idea I think: it would make the upgrade smoother for many users, and the same issue has been reported many times so far in Testing and Unstable, so it seems to be something used by a significant part of users. A bit too late to test out the fix though, as a lot of people already dealt with the file conflict by themselves.
An alternative would be to push a new version of
manjaro-system package that deals with the file conflict by deleting /usr/bin/dunstify. It would be a more “dirty” fix (despite being what some users did manually anyway), but it has been done in the past. It wouldn’t delete the old and now useless dunstify package though.
Both could be combined in theory, but that seems just overkill. lol
OT: Just wondering, what is it used for? Is it something people installs themselves or something included by default in some editions?