It seems that you need these icons for your project? Then it is better to define these icons in your own project, they are too special for Joomla core.

joomla-fontawesome icon classes are all refixed with fa- not icon-

Sorry @chmst you are mistaken

all of these icons are already present with the prefix fa- and not icon-

It seems that you need these icons for your project? Then it is better to define these icons in your own project, they are too special for Joomla core.

That's what I've done for now. I just figured that Joomla's best practice is to redefine icons.
Why are they needed then? When all of the fa- prefixes follow them.

joomla-fontawesome icon classes are all refixed with fa- not icon-

Sorry @chmst you are mistaken

all of these icons are already present with the prefix fa- and not icon-

I use these styles to add icons to menu items.
Tab Link Type -> Link Icon Class

The icon- prefixes works well.
The fa- prefixes don't work.

Okay, I found a way to use the fa- prefix. Standard icon family definition: fa fa-[icon].
This is even better because 'icon-[icon]' and fa fa-[icon] give different external results.
To make the display the same, it needs to remove the padding:

.p-2 {
   padding-left: 0 !important;
 }

But the question remains: why are icon- styles needed? And why do they not match the fa- styles specified below?

Hello maintainers, I’m interested in contributing to this issue. Kindly assign it to me so I can begin working on a solution and open a PR linked to this ticket. Appreciate your guidance! @SimonLitt

@SimonLitt Since Joomla comes with widely used standards but is supposed to remain very flexible, especially in the front end, it makes perfect sense to handle a healthy amount of standard icons with a general prefix in the styles which appear in almost every other icon set as well.

It is much easier and more efficient to customise one CSS/SCSS file than to find and overrite every single standard layout in which an icon is used.

This applies both to individual template creators who may want to use a different icon set tand to the CMS itself, should it in the futere become necessary to switch for example, for licensing reasons or similar.
I hope that makes sense.