Why does this work in Unity 2019 but not 2017?

BumpedSpecularSootNoKSP.shader

Shader "KSP/Bumped Specular Soot"
{
	Properties 
	{
        [Header(Texture Maps)]
		_MainTex("_MainTex (RGB spec(A))", 2D) = "gray" {}
		_Soot1("_Soot1", 2D) = "gray" {}
		_Soot2("_Soot2", 2D) = "gray" {}
		_Alpha1 ("_Alpha1", Range (0, 1)) = 0
		_Alpha2 ("_Alpha2", Range (0, 1)) = 0
		_Spec1 ("_Spec1", Range (0, 1)) = 0
		_Spec2 ("_Spec2", Range (0, 1)) = 0

		_BumpMap("_BumpMap", 2D) = "bump" {}
		_Color ("_Color", Color) = (1,1,1,1)
        [Header(Specularity)]
		_SpecColor ("_SpecColor", Color) = (0.5, 0.5, 0.5, 1)
		_Shininess ("_Shininess", Range (0.03, 1)) = 0.4

        [Header(Effects)]
		[PerRendererData]_Opacity("_Opacity", Range(0,1) ) = 1
			[PerRendererData]_RimFalloff("_RimFalloff", Range(0.01,5) ) = 0.1
			[PerRendererData]_RimColor("_RimColor", Color) = (0,0,0,0)
			[PerRendererData]_TemperatureColor("_TemperatureColor", Color) = (0,0,0,0)
			[PerRendererData]_BurnColor ("Burn Color", Color) = (1,1,1,1)
			[PerRendererData]_UnderwaterFogFactor ("Underwater Fog Factor", Range(0,1)) = 0
	}
	
	SubShader 
	{
		Tags { "RenderType"="Opaque" }
		ZWrite On
		ZTest LEqual
		Blend SrcAlpha OneMinusSrcAlpha 

		CGPROGRAM

        #pragma surface surf BlinnPhong keepalpha
		#pragma target 3.0
		
		half _Shininess;

		sampler2D _MainTex;
		sampler2D _Soot1;
		sampler2D _Soot2;
		sampler2D _BumpMap;

		float _Alpha1;
		float _Alpha2;
		float _Spec1;
		float _Spec2;

		float _Opacity;
		float _RimFalloff;
		float4 _RimColor;
		float4 _TemperatureColor;
		float4 _BurnColor;

		
		struct Input
		{
			float2 uv_MainTex : TEXCOORD0;
			float2 uv2_Soot1: TEXCOORD1;
			float2 uv2_Soot2: TEXCOORD1;
			float2 uv_BumpMap;
			float3 viewDir;
			float3 worldPos;
			float4 color : COLOR;
		};

		void surf (Input IN, inout SurfaceOutput o)
		{

			float4 color = tex2D(_MainTex,(IN.uv_MainTex));
			float3 normal = UnpackNormal(tex2D(_BumpMap, IN.uv_BumpMap));

			float4 soot1 = tex2D(_Soot1,(IN.uv2_Soot1));
			float4 soot2 = tex2D(_Soot2,(IN.uv2_Soot2));

			_Alpha1 = soot1.a * _Alpha1;
			_Alpha2 = soot2.a * _Alpha2;

			soot1.a = _Spec1;
			soot2.a = _Spec2;

			color = lerp(color, soot1, _Alpha1);
			color = lerp(color, soot2, _Alpha2);
			color = color * _BurnColor * _Color * IN.color;

			half rim = 1.0 - saturate(dot (normalize(IN.viewDir), normal));

			float3 emission = (_RimColor.rgb * pow(rim, _RimFalloff)) * _RimColor.a;
			emission += _TemperatureColor.rgb * _TemperatureColor.a;

			o.Albedo = fog.rgb;
			o.Emission = emission;
		    o.Gloss = color.a;
			o.Specular = _Shininess;
			o.Normal = normal;
			o.Emission *= _Opacity * color.a;
			o.Alpha = _Opacity * color.a;
		}
		ENDCG
	}
	Fallback "Standard"
}