I personally think so. I remember the hubbub around the Spiderman 2002 "twin towers" poster that went around for a while, and found that super interesting (the poster isn't lost, but it was discontinued and it IS expensive). Posters, like magazines, aren't typically designed to last long, making their preservation all the more meaningful.
Posters for the most part are used for advertising with art or some special graphic being specifically made for it, if the poster is lost then so is the art/graphic. So by that logic it is lost media.
