While IPS may not support FA5 yet as we have a lot to work on and consider there are ways to allow for this, in fact I personally use FA5 on my personal sites. It requires a minor edit to the custom.css and including the FA5 code. It would be beneficial if you allowed users to provide FA5 icons.
If you include the CDN or similar the code you need to make both FA4 and FA5 to work is:
.fa {
font-family: "FontAwesome" !important;
}
With this and including the FA import code FA5 codes will work as well as all FA4 icons currently included on IPS.