I have a 2000 with the thin headstock. My Dirty 100 also has a the same thin style, so I know they were at least doing them up until 2009. My 2016 McCarty (non 594) has the thick one.
I'm not sure what any of us consider thin and not thin. I brought my micrometer home from work and measured all the headstocks on non-fender and non bass inventory. They are in inches as follows (sorted by year):
1988 PRS CE24 = .6540
1995 PRS CE22 = .6240
1998 PRS McSoapy = .6035
2000 PRS SC = .6030
2002 PRS Cu24 AP Stoptail = .6495
2003 PRS Standard = .6755
2004 PRS SE EG = .6425
2004 Heritage H150 = .6015
2005 Heritage H150 = .6795
2006 PRS Cu24 AP = .6615
2006 PRS SC AP 20th Anni = .5995
2007 PRS CuRO22 = .6880
2007 PRS SE Soapy 2 = .6240
2008 PRS SC HB1 w/Piezo = .5860
2010 PRS Mira = .6275
2012 PRS SE Bernie = .6210
So for the analysis, my 98 McSoapy is thin but not the thinnest. The HB1 is the thinnest. The two SC's are thinner than the McCarty. The thickest is the Brazilian rosewood neck. One LP is slightly thinner and one is much thicker than the McCarty. The average is .633.
I think one could say the Kluson style tuner equipped models are thinner than the PRS locking tuner models, but the difference isn't an eighth of an inch, it is about one 20th of an inch. I think once they get done sanding and painting, the differences get mixed all around.