See comment above about the Equation Editor binary patch - they apparently lost the source code and it became too much of a security risk to keep (since it's missing all of the mitigations that a modern compiler generates).
Well, the result is the same. There're tons of old documents with formulas flying around and there's no clear path to upgrade, so the only reasonable way outside of re-typing every formula with new editor is to keep using old vulnerable editor. IMO they should have reverse-engineered old format and upgrade it to new automatically.
As far as I know, this was a component that was licensed from a third party. The company sold the editor separately as well IIRC. So, even if there are no legal hurdles to reverse engineering the file format, delving into an undocumented format developed by a different team and no previous information is tough. This means a lot of money and effort for something that comparatively few people really need. I am not surprised that they didn't do that.
https://support.office.com/en-us/article/equation-editor-6ea...