Comment by dragonwriter

They aren't interned frozen strings (unless they were symbols; String#intern was, and still is, an alias for String#to_sym, and String#freeze did not and does not imply String#intern or String#to_sym), and it also (even for literals) took an extra step to either freeze or intern them prior to Ruby 2.3 introducing the "# frozen_string_literal: true" file-level option (and Ruby 3.4 making it unnecessary because it is on by default.)

Amusingly, string literals interned by default in 3.4 or because of the setting in earlier >2.3 Rubies are still (despite being interned) Strings, while Strings interned with String#intern are Symbols.