I find exactly one .fa-spin definition in media\system\css\joomla-fontawesome.css. And also in media\vendor\fontawesome-free\css\fontawesome.css (the original!).

.fa-spin, .icon-spin {
  -webkit-animation: fa-spin 2s infinite linear;
          animation: fa-spin 2s infinite linear;
}

All other fa-spin finds define the animation.

@keyframes fa-spin {...}

• With fa-spin class you can make any icon rotate. if you add this class additionally.
• In contrast, the fa-spinner class has its predefined icon that will be animated depending on additional classes that define the kind of animation.

.fa-spinner:before {
  content: "\f110";
}

Are you sure that the fa-spin definitions are still needed ?

Yes. At least because of backwards compatibility.

@ReLater
Thank you very much for your detailed explication with which I'm been able to find the Font Awesome page explication

Can be closed?

Obviously not a joomla core issue. Thanks @ReLater