I see this tag on almost every manga. While I don't doubt there are people who use it as a troll because there defiantly are, I personally think there are people who use it as a proper filter. The tag could be a way to avoid reading/rereading stories that may seem interesting but aren't really for them. I don't personally use it myself, but I can see a few cases it might be helpful. Maybe they saw a misleading cover photo(ie thinking it's Yaoi/Yuri but it's crossdressing/gender bend or vice versa) when the story wasn't what they were looking for. Maybe they read it before but hated the direction the author took the plot(ie horrible/unsatisfying ending or they introduce an annoying trope/character they hate). Maybe the story is picked up by a terrible scan/translation team and they find it impossible to overlook. Maybe they saw tags/mentions of tropes they hate and/or find controversial/uncomfortable that aren't always obvious in the title/cover/description(ie incest, rape, teacher/student, abuse, or big age gaps) because not everyone is willing to read certain tropes even if it's well written or the art style is nice. In these cases, it's not really used to tell others "DO NOT READ" but more as a reminder to themselves to "DO NOT READ" a manga they already that already decided is not for them. While I understand the annoyance of a story I like getting labeled "DO NOT READ" and the tag driving away potential readers, I also understand the annoyance of getting into a story to not only to realize I already read it, but also that I hated it both times I did so
I have a feeling it'd be perfect for an Achilles x Patroclus fanfic where they reunite in the afterlife just like they do in the indie game Hades and are finally allowed to spend time together as lovers, as they always wanted to, but couldn't while they were alive.
Unfortunately, that would require me to actually WRITE the fanfic and anyone unfortunate enough to follow my works on AO3 will KNOW I'm not gonna finish it