As mentioned by Patrick, the IAU Working Group on Star Names (WGSN)
recently set about to formally catalogue star names, starting with the
brightest first.
The first official list came out in 2016 and there has been a handful of
revisions every year since then.
See
https://www.iau.org/public/themes/naming_stars/
The IAU decided to name individual stars rather than multiple systems.
Wikipedia cites a reference to "The Mythology of the Night Sky:
An Amateur Astronomer's Guide to the Ancient Greek and Roman Legends" by David E. Falkner
which says Girthab was an asterism formed by "Kappa Sco, Lesath, Shaula and Iota Sco".
https://en.wikipedia.org/wiki/Kappa_Scorpii
https://books.google.com.au/books?id...page&q&f=false
So with that in mind, it is likely the reason why Girthab isn't in the IAU
list because historically it was not a single star.