iOS 15: Works (iOS 15.2 beta 1)

A single line of JavaScript to apply a "black and white" filter to images. The result can be used as is or combined with the original image to get a partially desaturated version. As the example shows, the conversion supports images with alpha channel (transparency):

The shortcut converts silently any images received as input, giving them the same name/s.

When run without input, the shortcut works as a demo, allowing the user to select one or more files and showing the result of the conversion.

• No error checking
• globalCompositeOperation documentation at MDN

Don't download the shortcut

If you just want the code, there's no need to download the shortcut. Copy this and paste it in a "URL" action:

data:text/html,<script>with(document){let i=createElement("img");i.src="data:image/png;base64,REPLACETHISWITHBASE64IMAGE",i.onload=()=>{with(createElement("canvas")){height=i.height,width=i.width;with(getContext("2d")){drawImage(i,0,0),globalCompositeOperation="saturation",fillRect(0,0,width,height),globalCompositeOperation="destination-in",drawImage(i,0,0)}write(toDataURL("image/png").slice(22))}}}</script>

Replace the REPLACETHISWITHBASE64IMAGE fragment with the base64-encoded image, add a "Get File of Type" with type com.apple.webarchive below, and decode the base64 output to get the decolored image.

I use "Get File of Type" instead of "Get Contents of Webpage" to avoid that the user is misleading told that the shortcut uses Safari.