Resize macOS 8 & 9 icons
1) Open an icon in Affinity Photo
2) Choose 'Resize Document...'
3) Set the new dimensions and select 'Nearest Neighbour' as the resample method.
4) Use the Magic Wand to select the white space around the icon, and click delete to make it transparent. You might have to click the padlock in the layer to be able to do this.
5) Export the resized icon to .png
There, old school pixels in a modern container.
Orignal:
Resized to 1024 x 1024:
Original:
Resized to 512 x 512:
Original:
Resized to 128 x 128: