User blog comment:MeerkatQueen/List of VOCALOIDs with "br" phonetics/@comment-6433273-20190514162511/@comment-53539-20190514164118

Iroha has two vocals and both had them in V2.... So that theory may not be the best reason.

Yukari likely has them because she is a 3rd party developer group that simply use Ah Software for sales of their vocaloids, the same thing is true also for Nana. Ah software pretty much has adopted these vocals as theirs since their using them as a distributor, but technically only 3 Vocaloids have been developed directly by Ah software. Its similar to the Crypton Vocaloids and Internets.

Since there is quite a few voicebanks without them, its likely linked to the fact as of V3 you can insert samples into the Vocaloid lyrics. This replaces the need for BR as a separate phonetic, as you can insert them and more independently, quite a few Vocaloids did things this way.