well I would say there is a developement in tennis but we cannot say a certain year is the cutoff.
even in the 1920s there were some FHs that looked like a WW FH
http://www.youtube.com/watch?v=9IdvdxqSg8E
however there were also some very odd looking ones.
what is different now´is that the top100 strokes now all look virtually identical (apart from nadal maybe with his finish) because other strokes just cannot compete anymore.