Whenever MS introduces new features, they only apply to current & future releases, not to earlier ones.

This was true even with the .doc format. With Word 2000, for example, MS introduced the centring property for tables, which was not available in Word 97 (which require you to calculate your own centring). With Word 2010, MS introduced the checkbox content control, which is not supported by Word 2007, and so on. While Word 2000 and 2010 documents can be opened in Word 97 and 2007, they can't take advantage of formatting etc. created in the later version for which they never had any support.

The same is true of multi-column footnotes; they'll only be seen in Word 2016 and later (IIRC), not in Word 2013 & earlier. And, if you use Word 2013 & earlier edit a document that originally had multi-column footnotes, I'd expect that formatting to be lost when the document is subsequently re-opened in Word 2016 and later.

Microsoft helpfully includes a warning if you're about to take the document out of compatibility mode so as to employ a feature that isn't supported in earlier versions. Dismiss that, and you won't be able to use the feature.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]