In practice this kind of goes hand in hand with the series being in some way progressive, so currently the existence of an official content warning on a manga is itself a positive sign about the attitudes behind the series.
The most recent warning I've seen is a new series in Comic Beam called Tamon-san no Okashi na Tomodachi by Toi Yoh. It's a warning about allonormativity being depicted in the series, phrased in a way that effectively acts as a tip off that this is a story that doesn't think allonormativity is good and will depict it for the sake of examining it. Whereas the many manga that are allonormative are not going to have a warning for people who would want to avoid them.