It's not proprietary, it's still just plain text, and can be edited with anything. Emacs adds a lot of nice features on top of it to manipulate that text, but it's still just text.
As for extracting meaningful info with other editors, I would say it depends on how you structure your notes. For me it's just a very long bullet list with some tags here and there. So a plain text editor will only 'add' some star characters at the beginning of each lines, which appears quite readable to my eyes.
Obviously you won't be able to execute code blocks, reformat tables, follow links, fold sections etc.. but everything is just a search away..