When I use an Open Type font, like Myriad, the "Fractions" menu choice does indeed create fractions when there's a # / # configuration. But it also makes all the single numbers Numerators, including the punctuation. Very weird. (I recently upgraded to InDesign 15.0.2.)
I don't think it was intended to be applied to all of the text, but only to the portions of the text that are fractions. What would probably work best is to make a character style, and in the Open Type Features tab of the character style window, tick the Fractions checkbox. Then apply that character style to the fractions in your document.
I agree with Migintosh. You can't apply the OT Fractions feature to all characters. There is not way, for example, to distinguish a fraction (1/8) which needs a fraction layout with a ratio 1/8 which shouldn't be a fraction.
If you make a fraction Character Style you can apply it as a GREP Style with this expression:
This will change one or more digits followed by a slash, followed by one or more digits to your chosen Character Style.
This didi not work for me. There must be a better way.
> ... including the puncutation ..
FYI only: apart from the mere presence of OpenType Fractions ('frac') there is no standard between fonts in what the feature actually does. The "cheapest" option is to only replace "1/4", "1/2", and "3/4" with the proper (existing) characters "¼", "½", "¾".
With a bit more effort from the font creator, you can use any series of numbers to create fractions – but what it does depends on how the font creator implemented this. Some start off by changing all digits to superior, and then change (now superior) digits after a slash or fraction slash to subscript digits. This will always change all digits in selected text!
Others do a bit of intelligent lookahead and only change digits before and after a slash, but that still causes problems if you apply Fractions to a date notation such as "04/24/2020" ...
There is no consistency either in what characters are supported by the feature. Usually, and most often, it's just the digits; but as you found, Myriad Pro also supports decimal notation: "3.14159/4" will superscript the '.' as well, while Times New Roman will not.
Ultimately, what works depends entirely on the font. And indeed this should only be applied to what you want to fractionalize.