Hello,

The attached 2 patches add support for exporting unscheduled tasks and
repeating tasks to iCalendar, respectively.

For patch 1 (unscheduled tasks):

Currently, ox-icalendar does not allow creating an iCalendar task
without a scheduled start date. If an Org TODO is missing a SCHEDULED
timestamp, then ox-icalendar sets today as the scheduled start date for
the exported task.

Patch 1 changes this by adding a new customization
org-icalendar-todo-force-scheduling. When non-nil, the start date is set
to today (same as the current behavior). When nil, unscheduled Org TODOs
are instead exported without a start date.

I also propose the default value to be nil. Note, this is
backwards-incompatible with the previous behavior!

But I think it should be the default anyways, because IMO it is the more
correct and useful behavior. An iCalendar VTODO without a DTSTART
property is valid, and has the same meaning as an Org TODO without a
SCHEDULED timestamp. Also, all the iCalendar programs I have tried
support unscheduled tasks, including Thunderbird, Evolution, Nextcloud,
and Tasks.org.

For patch 2 (repeating timestamps):

I add recurrence rule (RRULE) export for repeating SCHEDULED and
DEADLINE timestamps in TODOs, similar to how repeating non-TODO events
are currently handled.

The main complication here is that iCalendar's RRULE applies to both
DTSTART and DUE properties; by contrast, Org's SCHEDULED and DEADLINE
timestamps may have different repeaters. I am not sure the best way to
handle the case where SCHEDULED and DEADLINE have different repeaters,
so in that case I issue a warning and skip the repeater.