Letters to the newspaper have been a feature from the start; a means to connect with the community. The main business of a newspaper is to relay the things happening in their area of reach. While the other paper media don't need to identify with a region, it is what newspapers do.
Letters pages are also a staple (or the pages either side of the staples) of magazines.
I haven't sampled enough trade journals to know whether the readers of Knife Sharpeners Odyssey write in and exchange thoughts on their vocation. Generally academic journals are solely comprised of peer-reviewed pieces, not random contributions from scholars.
I've been saying that newsletters are short and to the point but the definition - as with the play between 'magazine' and 'journal' - is not as hard and fast as all that. Though a search seems to indicate that the newsletter, as the name suggests, is the letter.