we added to the toolkit (creation and upgrade processes) support for font-awesome, what was before in classic and admin themes in portal. This is in 9.3.0 version.
Font-awesome and glyphicon (liferay-font-awesome) is included in classic and admin themes because the build process includes it (because the new toolkit version). That's what we removed from portal, that custom integration, now comes with the standard process
When upgrading a theme user is prompted if he wants to include font-awesome (yes by default)