Zero-width space is currently not mentioned in (e.g.) https://spec.commonmark.org/0.29 at all.

However, it's suggested to be used for certain cases. E.g.:
commonmark/commonmark.js#108 (comment)
commonmark/commonmark.js#148 (comment)

There are also cases where it works for single indicator of inline structure, but not for double:
commonmark/commonmark.js#174

Hence, IMHO, it's worth documenting this.