//From https://ax23w4.itch.io/lightrays-2d-effect
 
Shader "Custom/LightRays"{
    Properties{
        _Color1("Color1",Color)=(0.1,1,1,1)
        _Color2("Color2",Color)=(0,0.46,1,0)
        _Speed("Speed",Range(0,5.0))=0.5
        _Size("Size",Range(1.0,30.0))=15.0
        _Skew("Skew",Range(-1.0,1.0))=0.5
        _Shear("Shear",Range(0.0,5.0))=1.0
        _Fade("Fade",Range(0.0,1.0))=1.0
        _Contrast("Contrast",Range(0.0,50.0))=1.0
    }
    SubShader{
        Tags{
            "Queue"="Transparent"
            "IgnoreProjector"="True"
            "RenderType"="Transparent"
            "PreviewType"="Plane"
        }
        ZWrite On Lighting Off Cull Off Fog {Mode Off} Blend One Zero
        Pass{
            Blend SrcAlpha OneMinusSrcAlpha
            CGPROGRAM
            #pragma vertex vert_img
            #pragma fragment frag
            #include "UnityCG.cginc"
 
            float4 permute(float4 x){
                return fmod(34.0*pow(x,2)+x,289.0);
            }
 
            float2 fade(float2 t){
                return 6.0*pow(t,5.0)-15.0*pow(t,4.0)+10.0*pow(t,3.0);
            }
 
            float4 taylorInvSqrt(float4 r){
                return 1.79284291400159-0.85373472095314*r;
            }
 
            #define DIV_289 0.00346020761245674740484429065744f
 
            float mod289(float x){
                return x-floor(x*DIV_289)*289.0;
            }
 
            #define REPEAT_Y 1000.0
 
            float PerlinNoise2D(float2 P){
                //Mirror the pattern on large Y number
                if(floor(P.y/REPEAT_Y)%2==0){
                    P.y-=floor(P.y/REPEAT_Y)*REPEAT_Y;
                }else{
                    P.y-=floor(P.y/REPEAT_Y)*REPEAT_Y;
                    P.y=REPEAT_Y-P.y;
                }
                float4 Pi=floor(P.xyxy)+float4(0.0,0.0,1.0,1.0);
                float4 Pf=frac(P.xyxy)-float4(0.0,0.0,1.0,1.0);
                float4 ix=Pi.xzxz;
                float4 iy=Pi.yyww;
                float4 fx=Pf.xzxz;
                float4 fy=Pf.yyww;
                float4 i=permute(permute(ix)+iy);
                float4 gx=frac(i/41.0)*2.0-1.0;
                float4 gy=abs(gx)-0.5;
                float4 tx=floor(gx+0.5);
                gx=gx-tx;
                float2 g00=float2(gx.x,gy.x);
                float2 g10=float2(gx.y,gy.y);
                float2 g01=float2(gx.z,gy.z);
                float2 g11=float2(gx.w,gy.w);
                float4 norm=taylorInvSqrt(float4(dot(g00,g00),dot(g01,g01),dot(g10,g10),dot(g11,g11)));
                g00*=norm.x;
                g01*=norm.y;
                g10*=norm.z;
                g11*=norm.w;
                float n00=dot(g00,float2(fx.x,fy.x));
                float n10=dot(g10,float2(fx.y,fy.y));
                float n01=dot(g01,float2(fx.z,fy.z));
                float n11=dot(g11,float2(fx.w,fy.w));
                float2 fade_xy=fade(Pf.xy);
                float2 n_x=lerp(float2(n00, n01),float2(n10, n11),fade_xy.x);
                float n_xy=lerp(n_x.x,n_x.y,fade_xy.y);
                return 2.3*n_xy;
            }
 
            float4 _Color1;
            float4 _Color2;
            float _Speed;
            float _Size;
            float _Skew;
            float _Shear;
            float _Fade;
            float _Contrast;
 
            fixed4 frag (v2f_img i):SV_Target{
                float4 color=lerp(_Color1,_Color2,1-i.uv.y);
                float noisePos=i.uv.x;
                noisePos+=-0.5;
                noisePos*=_Size;
                noisePos+=(1-i.uv.y)*(_Size*_Skew);
                noisePos*=1/lerp(1,_Shear,1-i.uv.y);
                float val=PerlinNoise2D(float2(noisePos,_Time.y*_Speed)) / 2 + 0.5f;
                val=_Contrast*(val-0.5)+0.5;
                color.a*=lerp(val,val*i.uv.y,_Fade);
                return color;
            }
            ENDCG
        }
    }
}[/code:ihoa70sn]

Thanks very much for you help, Gigatron !
Now it works as a light rays :
[img="https://s7.postimg.org/3uvkrg9uj/Capture.png"]
But it's a little weird compared to the original result :
[img="https://img.itch.zone/aW1hZ2UvMTI4MTEwLzU4OTY1OC5naWY=/347x500/ll8oV5.gif"]
And in C2, it's pink...