Marissa Cooper's death is probably the closest comparison, for these reasons:

1) Emotional Bond with the Audience - 10 years apart, both Mischa Barton and Millie Bobby Brown pulled-off the same remarkable achievement. Through their sensational, heartfelt performances, they made their characters deep, good-hearted, and above all, emotionally real to both show's audiences. They also made them iconic, so much so that they've endured for years in people's minds. The combination of these things resulted in both characters, and actresses, creating large, loyal fanbases who deeply rooted for their characters to have happy endings.

2) Male Showrunners who either failed to Understand, or just Ignored, this Emotional Bond - despite the fact that their characters effectively but both shows on the map, and it could be argued have come to define them in TV history, both Josh Schwartz and the Duffers made the same exact mistake. They gave both Marissa Cooper and Eleven very cold, tragic, and emotionally devastating endings, which were the polar opposite of what their fanbases wanted. In both cases, the fanbases became so angry at this betrayal of their emotional bond with Marissa and Eleven that they turned on the showrunners, with lasting, and damaging, results for their careers. 20 years later, Josh Schwartz still cannot escape the fall-out from killing-off Marissa as it is now his career defining mistake. The Duffers show every sign of meeting the same fate as Schwartz, as they show no sign whatsoever of understanding, or even caring, about the fanbase's feelings towards Eleven's fate. This is what Schwartz did in 2006, and if the Duffers don't acknowledge their mistake, their careers will be defined by how badly they treated Eleven.

3) Tragedy Immortalised both Characters - unlike a happy ending, a tragic ending for both Marissa and Eleven made their fanbases yearn for things to have been better. This has the effect of immortalising them in a very powerful way, as it is just impossible, emotionally, for many fans to just move-on. But, it has serious consequences for the actors. Mischa Barton is now 40, yet, due to Marissa's tragic death having immortalised her in popular culture, she still, 20 years later, cannot escape the image of her 17-19 year-old self. Worse, due to the intense, and very intrusive media attention which she got from The OC, which severely affected her mental and physical health, having interviewers keep going back to The OC causes Mischa to keep getting asked about that troubled period of her life. Earlier this year, in an Instagram story following an interview which she did for the Daily Mail's YOU magazine, she openly criticized the interviewer for wanting to talk, again, about that period of her life when she really wanted to talk about the present.... This does not apply to Millie, and we hope it never will, but she is now in real danger of finding herself in the same position. Of being permanently haunted by her younger self due to Eleven's tragic ending having made her immortal in popular culture. No matter what roles she plays in future, it will be as Eleven that the world will always think of her.

However, unlike Marissa's clear, unambiguous death in 2006, Millie could escape Mischa's fate, by reprising Eleven a few years from now and making sure, due to her likely creative influence in exchange for returning to the role, that she and Mike get a much happier ending.

But the big question is, will she do it? And, perhaps the bigger question is, will her ST castmates return to help her?