What happened to NS spectral types after OBAFGKM?
They still exist, but are perhaps less used because they don't indicate a particular temperature, or mass.
The stellar classification is based on the presence of particular lines in the spectrum:  A type stars have a strong hydrogen line. B type stars have Helium.  F and G  and K stars have various ionised or neutral metals. M type stars have compounds like TiO2 in their atmosphere.
But these are associated with temperature, and B type stars are hotter than A type (and O type stars are hotter still) hence OBAFGKM.
Asymptotic branch supergiants can have processes that dredge up carbon from their interior, producing carbon lines.  These are the R and N type stars, but R type stars are similar in temperature to K type stars. and N type are like M type.
S type stars have heavy elements produced by s-process neutron capture stars.  So if you are ordering the stellar classification by temperature, these don't have a natural place to go, and since the purpose of &quot;Oh Be A Fine Girl Kiss Me (Right Now Sweetheart) is to remember the order of the letters then RN and S don't fit, since they are not ordered by temperature.
As for political correctness, many alternatives have been suggested, but none has caught on, perhaps because a good mnemonic should be a bit &quot;naughty&quot;.