An alternative to @Rahul's suggested fix:
Plot3D[Sin[x+y^2],{x,-3,3},{y,-2,2}, PlotTheme->{"Classic","ClassicLights"}]

The associated Lighting
setting matches the one in Rahul's post:
"DefaultLighting"/.(Method /.Charting`ResolvePlotTheme["ClassicLights", Plot3D])
(* {{"Ambient", RGBColor[0.312, 0.188, 0.4]},
{"Directional", RGBColor[0.8, 0, 0], ImageScaled[{2, 0, 2}]},
{"Directional", RGBColor[0, 0.8, 0], ImageScaled[{2, 2, 2}]},
{"Directional", RGBColor[0, 0, 0.8], ImageScaled[{0, 2, 2}]}} *)
Update: Few more combinations of options that give the same result:
Plot3D[Sin[x + y^2], {x, -3, 3}, {y, -2, 2}, PlotTheme -> "Classic", Lighting -> "Classic"]
(* thanks: Algohi *)
Plot3D[Sin[x + y^2], {x, -3, 3}, {y, -2, 2}, Lighting -> "Classic", PlotTheme->None]
PlotTheme->"Classic"
should be settingLighting->"Classic"
to get the old lights. $\endgroup$