I believe everything listed here is available for use: Find the Perfect Icon for Your Project in Font Awesome 5 | Font Awesome
Though some icons require an additional step… for icons we don’t use in Discourse by default the icon names needs to be added to the Svg icons theme setting.
If you really want to be specific, all the icons we use in Discourse by default are listed here: discourse/svg_sprite.rb at da0d20d4a99aab40bd08bf133a425be413521d5a · discourse/discourse · GitHub
And all the icons available for use in themes (via the svg icons setting) are in this directory: discourse/vendor/assets/svg-icons at main · discourse/discourse · GitHub
Viewing the SVG files in github isn’t really helpful, because you’ll see a big white box:
but you can click the raw button and view the page source, the icon name is the symbol’s id
