They were added in 4.3.0 “Hour of Twilight” which is the Dragonsoul patch in cata. Even the Blizzard posts and wowhead states this:
I have the mog since WoD, which isn’t fairly recently. Your memory from 12 years ago is wrong. You can stop trolling now.
Why are you messaging me? Stating facts, like them or not.
There have been reports of people being 1800 rated bg and can also NOT mog them. They are literally just not marked to mog yet because the company does not care.
Soon TM i guess
grabs ticket for waiting room
Not fixed/added yet
Taps foot patiently
Bump…
Blizzard…
still no r14 gear
Still nothing 
still nothing no r14 comon
Day 69604348 - Classic PvP transmog not added yet…
yes bring it baby
2 weeks later, still no fix in sight…
People who’ve earned the rank before? No.
People who haven’t earned the rank? Also, no.
They said even people without the rank could Transmog them, but that was wrong.
It was tho.
And also, they wanted everyone even those without rank to be able to mog them.
They just haven’t done it correctly.
Even those with current RBG rating cannot transmog it haha, no1 can
Yeah, original Warlord rank? Nope.
No rank back then? Nope.
Current RBG rank? Also, no.
They really screwed it up, massively.
Tomoz, It will be fixed right(In our dreams)
It wasn’t tho
One day it will be fixed…