I have proposed this in an email suggestion to Medium before as well; some essays (here and elsewhere) reflect a blatant mental health issue presented as "psychological" or "spiritual" advice, or they promote ideologies that threaten physical and mental wellbeing (e.g., misogyny, toxic masculine emotional suppression). It's not enough for a platform to decline to promote such pieces; it's important to flag them with a disclaimer, so that readers who stumble upon them understand that there is something unhealthy in the message.

That said, I'm not sure how to implement a feature like this without the possibility that bad-faith actors would abuse it (e.g., flagging harmless essays as "dangerous" simply because they want to mute the author). It's a tough question, but social media platforms have a HUGE impact on mental health today, and it will be so important for them to develop policies that account for this.