Shiny’s icon() function only supports Font Awesome 4.7 icons. A full list of the icons supported by Shiny can be found here.
Since then, Font Awesome has been updated many times with new icons. How to use Font Awesome 5 icons in Shiny apps, or how to upgrade Font Awesome in Shiny to version 5?
Adding the following line to your ui.r file will expand Shiny support for Font Awesome 5.7.2 icons:
tags$style("@import url(https://use.fontawesome.com/releases/v5.7.2/css/all.css);"),
Afterwards, you can use the function icon("icon-name") in your Shiny web app. For example, you can use icon("vote-yea") to check if these new Font Awesome icons are working as intended.