I also had this issue. This morning, I updated WordPress from version 5.6.2 to version 5.7, and I updated your plugin from version 1.7.4 to version 2.0.0. When I looked at my website, all of the icons had broken. At that point, I noticed that your plugin says that it has only been tested up to WordPress version 5.6.2, so I figured the update to WordPress version 5.7 had broken the plugin. At that point, I reverted back to a backup file.
When I went through the second time, I left WordPress on version 5.6.2, but still updated your plugin to version 2.0.0. However, that also broke the icons on my site. It was at this point that I did a little research and learned that the “fa” style prefix was deprecated in version 5 of Font Awesome. Maybe not so coincidentally, your update to version 2.0.0 included support for version 5 of Font Awesome. Is it possible that adding this support broke backwards compatibility with version 4? If so, that would mean I’d need to recode every instance of an icon that uses the “fa” style prefix, which is all of the icons on my site.
At this point, I’m thinking the issue is one of two things…
1) Version 2.0.0 of your plugin is broken in some way that I don’t understand. But whatever it’s doing, the icons on my site have stopped working.
2) Version 2.0.0 of your plugin is working, but only for version 5 of Font Awesome. If this is the case, fixing the issue would mean manually updating the code of every icon on my site.
Whether #1 or #2 is correct, I’m no longer concerned with. Even if #2 was correct, I don’t want to spend the time manually updating the code for every icon on my site. Instead, I switched to the WP Font Awesome plugin, which still supports the “fa” style prefix from version 4.
Although I’ve deactivated your plugin for now, it has worked well for us for years, so I will check back a few times to see if this issue has been fixed.