Varies by guitar. There is no line in the sand.
I've refined the thread to consider only Starfires. It could be applied to other models/families but one can start somewhere.
Nut width, surely, is a policy decision. It could well have changed following the Fender takeover. Narrow nuts seem to be consistent from the '60s all the way through to the late '90s, but it would be useful to gather some data. Perhaps we could have a LTG nut width poll?
It's interesting that people carp endlessly about Rickenbackers being hard to play due to a narrow nut, but it's rarely discussed in relation to Guild - perhaps because Rics are known to many for 12 strings.
The 1997 discussed
here has a 1.63" nut, as does
this 1994. These match my 1989 X160.
As documented by GAD, when the pups change is variable, and could well be related to what stock was in house. I'm thinking that if the SD1s come in from about 1997, it may not be possible to have a wide nut with HB1s, although you could retrofit them until the pups got smaller a few years later on.