Yeah, the thumbs down on FontAwesome is a little harsh imo. Maybe there should be a thumbs down on FontAwesome using the CSS classes flavor, but I don't see the new version as being a bad thing at all.
The thing is there are so many good reasons to use an SVG icon font system nowadays: pure vector, control over the individual shape symbols, positioning is easier thanks to knowing its exact size etc.
It's just nitpicking; with the example it's pretty clear that it's the icon font that is bugging you.
Icons are overused, there's no onboarding of the UI and it just throws you in with a bunch of icons, I wasn't sure what most of them did.
I started at mastodon.technology and it immediately follows the instance creator which confused me, when I checked his profile there was an icon with which I could unfollow(an X on top of a person icon), which I didn't know if it meant blocking or unfollowing.
No icon has helper text on hover or anything, you're just thrown in and there's already messages popping in your timeline and I didn't really know that I already followed these people, and I could click Local/Federated timeline and there wasn't much info on what those were.