> Past events: date and time string in UTC with UTC time zone, unless there is a strong case to be made that these should be localized (see “planned local events” above)

I don't think there's any case for storing local time with past events. They happened at a specific date and time, store them in UTC and if needed convert to local on display. Plus, though it's a rare circumstance, for one hour each year DST switches cause a duplicate hour - so local time is ambiguous during that hour.