I think lesbian death happens more in original media sources; at least the U.S. ones. Japanese media tends to portray f/f (yuri) as a passing phase, and how het is the only "real" type of relationship.

Yep, lesbianism=death is very common in western television, and within fandom, that is often represented as lesbians not getting a HEA (I've edited my post to clarify this.)

They have that in common, but m/m and f/f do operate differently. M/m slash and het can put the female gaze on men, which is the opposite of how it usually works in the mainstream. Het and f/f slash can be about women writing about female characters.

That is a very good point.