Before Rails 5, we could fetch all time zones for US by using us_zones method as follows.
Such functionality of getting all the TimeZone objects for a country was implemented only for one country, US.
The TimeZone class internally uses the TzInfo gem which does have an api for providing timezones for all the countries.
Realizing this, the Rails community decided to
introduce a helper method country_zones
to ActiveSupport::TimeZone class that is able to fetch a collection of TimeZone objects
belonging to a country specified by its ISO 3166-1 Alpha2 code.