If I publish a .ics file for purposes of being subscribed to by a calendar application (Google, Outlook, etc), do I have to keep events that have passed? Or can I only include upcoming events?
The ICS file doesn't require old events be kept. If you delete old events from the ICS file, they would cease to exist in the user's calendar, since the ICS file is distinctly separate from the recipients calendar. Most users would probably be comfortable displaying events only going forward.