When a normal citizen (so not a nobel or rich family, say, farmers for example) went to war (forced or freely), and then they died in said war, was his family informed of that fact, in one way or another? If yes, how? I'm interested in the European medieval age, and the roman republic specifically, but not exclusively.
Not on any organised scale. Death notification is a newfangled introduction as far as warfare goes. Further, there must be a caveat present here: often we don't know a lot about commoners, 'normal' people, the subaltern; the vast majority of those voices are lost to us, as observed beneath. More can always be said on the matter, so if you'd like to speak more about pre-modern military death notifications, please feel free to post up!
For the meantime, OP, we do have some previous posts for your attention.