While I’d certainly love to see the Font Awesome library that’s included be updated to the latest version (then updating the available icon set to select from accordingly), there is technically a workaround in the meantime.
I found the best approach is to first make sure the site is loading a modern version of Font Awesome that includes the new icon (ex. use https://www.ads-software.com/plugins/font-awesome/ to load that up-to-date FA library version which then has a Troubleshoot tool that can then help identify & prevent any other FA versions from being loaded to avoid conflicts & unnecessary assets being loaded on the site [as an aside, it would be great if Menu Icons had a readily available option to have it not try to load its own Font Awesome icon library on the public pages, while then still enabling that as the icon set to use for a menu, for if/when FA is being loaded elsewhere on the site given how popular that icon set is while Menu Icons is commonly not the most up-to-date option for those assets while it could/should be able to just use what’s already being loaded elsewhere for its own icons, at least for outside of the site admin.])
At that point, the Menu Icons Settings section when editing a menu offers a place to include custom CSS classes which then adds those comma-separated classes to the icon picker once saved. The CSS classes I’ve then added to accommodate this is:
fa-brands fa-square-x-twitter,fa-brands fa-x-twitter,fa-brands fa-mastodon
(I added 2 X icon variations as well as adding Mastodon which was also missing in that older version bundled with Menu Icons… customize to use whatever classes the Font Awesome site documents for a particular icon [making sure the fa-brands/fa-regular/etc. class is included right alongside the icon name like fa-square-x-twitter/fa-mastodon/etc.])
Again, it’d be great if the built-in FA library gets updated to be current while also having it so Menu Icons can allow Font Awesome to be enabled for a menu while then opting to not have Menu Icons try to load its version on the main website (instead delegating that to a more dedicated/up-to-date offering like the official Font Awesome plugin or similar.) That way, there’s no conflicts nor duplicate assets being loaded (also then not needing to have the theme or a plugin then check & remove the duplicate and/or older version of FA being loaded by Menu Icons since Menu Icons just follows that setting to simply not include it when set) while the icon picker can be given a push to include new icons before they’re officially added in a future plugin update via the custom CSS classes field it already has. That way, sites get the flexibility to add new icons & has an overall improved experience while this plugin then doesn’t need to keep releasing new versions anytime Font Awesome gets a new version (while it’d still be good to stay somewhat up-to-date to reduce the reliance on custom CSS classes and/or needing a separate FA version being loaded from elsewhere to get a particular icon for more basic setups.)
