I know you're trying to keep this limited to Affinity Photo but it's the one feature AD is lacking for me. I work designing websites for e-commerce and I often need to cut out product images from white backgrounds. The work isn't complicated enough to require a dedicated piece of photo software like AP so I'm not going to spend an additional 40 quid for it. I'd like to see it added to AD.
This feature is literally the last thing that's keeping me from removing Photoshop completely from my mac.