Thank you, creedon. Thank you, tuanphan.
I'm using psuedo to apply icons in other parts of my design. But having the ability to drop different icons into specific buttons was very helpful. I know I can use CSS and unicode to target individual buttons, but doing so for a range of buttons across multiple pages would be a pain to create and manage.
Squarespace support would neither confirm nor deny that they closed the loophole. They only told me they can't help with custom code. But talking to support at Font Awesome, they claimed to be aware of other customers using the same method to insert icons into buttons on Squarespace. It seems strange that Squarespace would intentionally remove functionality that supports such a hugely popular and easy-to-use icon service. (Unless, maybe, Squarespace was planning to acquire or partner with some alternate icon provider.)