Curious how this affects date/time algorithms and software libraries, particularly those that do not have a way to sync with an external authoritative source of time. I guess they all need to be updated to account for this change?

They need to be updated anyway, and always have been: somewhere in the world, someone changes time rules basically every year, or even every few months. All current OSes already have support for stating "locale X will not use daylight savings / will always use daylight saving, starting from day yyyymmdd at hhmm" through simple updates.

Yes there’s databases like https://en.wikipedia.org/wiki/Tz_database that track this and that are included in libraries that do the arithmetics. It’s why those libraries are often relatively bulky.

An updated tz database.