A thought I had re: her height and how skinny she was — could she have had Marfan's Syndrome? I had a friend who had it, and along with being ridiculously tall and skinny, they also had spinal issues (although in their case it was scoliosis rather than spina bifida occoulta). Not sure if there's any way to diagnose that post-mortem, but it was just something I was thinking.
Additionally, could she have been adopted? If she was adopted out from another family (possibly even one in Canada — given her estimated birthdate, it's possible that she could have been a part of the Sixties Scoop — perhaps she was a particularly dark-skinned Indigenous girl or even biracial; though black children were also over-represented in the Canadian welfare system at the time), then it would explain why no one has recognized her, particularly if her killer was part of her adoptive family (though it's also possible that her adoptive family just didn't care enough to report her missing). It's a common theory that Hope Doe wasn't from St. Louis, so it's remained a question as to how she ended up there. If she was adopted, perhaps she was trying to return to her birth family, like it's reported several of the children adopted out in the Sixties Scoop did? If this is the case, it's possible that her birth family would not know about her death, or her Marfan's, and could still be looking for her, but at the age that she would be today (roughly mid-40s). I'm not sure how one would go about getting adoption records from that time period unless you're related to someone involved, but if anyone knows, please share — I'd love to look into this further if it means this little girl could finally be identified.