Why is “vedicAnusvara-beng” a letter and “anusvara-beng” a spacing mark?
This is based on the character’s Unicode properties. The requester hasn’t provided sufficient rationale for the properties in their proposal (page 2). In fact, they suggested that, “[…] the character (and other similar characters for Vedic usage) should be treated equivalent with the anusvara, since it only occurs in place of the latter in specific contexts.” which is …interesting.
Anyway, it’s only used in the reproduction of classical/ancient texts, so not seen at all in contemporary use.