wpForo uses Font Awesome 5.0.6 version. I'm sorry but there is no way to update it. We're updating the core time to time. The latest FA version will also be added in wpForo soon.
Same problem here. I wish there was a custom link to what's really included in wpForo, instead of that link to FA website with more than half the icons there not displaying in wpForo.
Honestly, I think this is really just one of those little things. Especially when you consider all of other great features and outstanding support for free software.
What I'd really like to see is the ability to use any image we choose instead of being locked into font awesome. But again, this is a rather small thing.
I don't think the 5.0.13 version got more than half new icons after 5.0.6. Please don't write things that are far from truth. You just need to check the version of icon before using it. The new a icons are not so many.