You need a Pro FA5 account. These were available for only $20 during the early crowd funding FA5 phase, but I think they are $99 per year now. That $20 bought you a lifetime Pro membership amongst other things and allowed you to download the whole FA5 set in icons in light, medium, solid and brands, and just for the SVG versions, was a great deal for $20. With the Pro membership you can download the Subsetting tool from the FA Downloads page (but must enable beta downloads, to see the download).
However, I find myself not using FA icons much or hardly at all, these days. I prefer to use SVG images for larger page icons and the only time I really need to embed an FA icon is usually or bullet points or accordion arrows. It makes little sense to download all 7842 FA5 icons, and better sense to just download the ones you want, to speed up page loading.