EvgeniySotnikov I have found the reason, but I'm actually not sure whether this was intentional in Flarum.
Flarum currently requires version 5.9.* of FontAwesome, meaning you can only use icons present in the 5.9 releases.
On FontAwesome website we can can see the Shopify icon was added in version 5.12.1 https://fontawesome.com/icons/shopify?style=brands . As such, it's not available.
In the past it was possible to manually update FontAwesome independently from Flarum to get access to the latest icons, but with the change of requirements introduced in flarum/core1790 it's now impossible to update FontAwesome past version 5.9.
@luceos do you know if the change from ^ requirement to .* was intentional (you authored the commit, but I can't find any explanation behind the .*) ? If this was unintentional, we should replace 5.9.* with ^5.9.0.
