There are quite a few popular speakers in mastering studios: Dunlavy SCIII/SCIV, etc., B&W 801, K&H O300, Lipinski, and most recently Tyler Acoustics. However, mastering engineers, like any other audio engineer, use what works for them. The NS10 only became popular because it did the job for not a lot of money at the time. There are so many better options these days.
At abbey road they do have NS-10 in their mastering rooms.
I do have a pair in the studio I dont use them very often any more but they did give me a good idea of what my client heard during mixing (if they used ns10s). so i had a bigger urstanding.
I use b&w 801 and b&w 805 now.