I don't know if this is the right place for this so please feel free not to publish this if it feels too heavy, but it feels relevant to what you're talking about.
It absolutely drives me up the wall the way people cling to having "mdni" in their bio as of it is something helpful to "protect children" but even more often, it's there to feel like they are protecting themselves. From the ages of 14-17 I was heavily groomed on Tumblr, and used it as a platform to do sex work. Whenever I see mdni or 18+ on someone's blog, I think about the way that almost every single one of the people I spoke to, sold to, even met up with, had this displayed prominently on their blogs. It was there entirely as a facade to keep their upstanding image of being against child abuse, or to dampen the possibility of legal issues, while privately preying on children. I would never reach out to someone who had this in their bio, it was always them reaching out to me.
I think of the amount of blogs that I saw where people were talking about their experiences with CSA, and the ways that they were now engaging in sex in ways that were healthy and healing, the people who could've understood what I was going through, may have been able to help me, but I never reached out to ask for help because they had "mdni" in bio and I didn't want to break their boundaries.
On the flip side of this, the very few people who ever saw my blog and what I was doing and reached out to me, to ask me if I was okay, to try and mitigate the danger I was putting myself in doing sex work at 15, never had mdni or 18+ in their bio.
I don't know, I think it's just very bleak and unhelpful that this is the standard way for people to operate. Obviously I think it's completely fine to not want to engage with people under 18 when your blog is centered around sex and kink and not have sexual encounters with them, but as and adult I've found it incredibly easy to simply block someone if a situation like that arises, or reach out and see if you can help them.
Putting "mdni" in your bio is instead putting a sign at the door that says "if you are a child there is no place for you to engage with sexuality in a way that is healthy, and you have no one to turn to." It doesn't protect any children, it makes them more unsafe, and lets you wash your hands of doing any of the work of actually helping children, be that directly or by being a place where they can learn.
Posted without additional comment because I truly have nothing to add, this says everything already
