I've seen some sketchy crap while pulling apart mac .pkg files to see their preinstall/postinstall scripts. In particular one video conferencing company's installer did some "growth hacky" things a few years ago (I checked a recent one just now and it seems benign).

Ah yeah I remember those particular installer shenanigans for sure. Indeed, installers are often granted elevated permissions which is a perfect opportunity to drop in "extra" functionality :-O