As someone who fanatically enters [none] barcodes for digital releases, I disagree with leaving them blank.
First of all, do digital releases really have a "barcode"? From what I've seen so far, most have none or only a catalogue number at most. If there is a barcode field, is this only for compatibility with physical releases?
Secondly, do they have the same barcodes on different stores, e.g. iTunes and Spotify or are those automatically generated by the respective store? If the latter is the case, wouldn't this require creating a release per-store just for the barcode?
And last but not least, regarding whether to use [none] instead of blank barcodes, I tend to look at and add links to all online stores selling a certain release and shouldn't the fact that none of them display a barcode be enough to confidently set the barcode to [none]?
Anyway, just my two cents, but in any case I think the style guideline should be updated for digital releases as the current one does neither acknowledge the existence of [none] nor it's appropriate use.