Supporting FontAwesome 6 in core is difficult, since it uses Less features which the (very outdated) Less PHP library we use does not support. As a result, the only way we can implement support is with a second or new styles compiler (probably SCSS), or by manually editing the files to use older syntax.
Editing FA like this isn't something we'd like to do within core, as it makes it much more difficult to update to new Font Awesome versions in the future, so migrating to SCSS or using it alongside Less is our only real choice, unfortunately, which is almost certainly a breaking change.
But such a 'hack' is more acceptable within a Flarum extension, so Blomstra have released an extension to enable Font Awesome 6 Free and Pro support. FA6 Pro is only supported through their kits service, as we can't easily redistribute FA6 Pro within the extension, but I have ideas for how we could let people upload their own copy of FA6 Pro they have downloaded so this isn't a limitation in the future. No promises about when this might be, though.