I'm thinking Apple has a software problem. Public holidays are known years ahead.
All of their apps have a built-in Holliday calendar you can show from the preferences, but those are different than the ones you can subscribe. They go out to end of 2022. They may have finally dropped support for the other calendars.
Since the built-in Holliday calendars include a bunch of garbage that is of no use to me as a "Holliday," I created my own on iCloud and subscribe to that on all of my devices. Most Hollidays can be created using the repeating tools provided, but those that are based on lunar activity require you manually enter them. It takes maybe 30 minutes to set up your own custom calendar that has the entries you want to see and none of the useless stuff (including going out several years with lunar-based dates). I just used the calendar in Terminal to figure out those for about five years out.