to be honest, some all-day-events might not be relevant to block the availability. But those events which are in a busy-status shall.
So, maybe the availabilty status for events busy, tentative, available should be able to be set by the user ...
Then, those all-day events in busy status can block availabilty as well.

Indeed, this is crucial to avoid getting booked when on a multi-day trip for instance.

The appointment booking feature should treat full-day and multi-day calendar events as unavailable time.

At present, it appears to ignore:

Full-day events, such as an event covering all of Monday (as described above)
Multi-day events, such as an event running from Monday at 14:00 until Wednesday at 10:00.

This means clients can book appointments during periods that are already occupied in the calendar, creating double bookings.

Before the feature can be used reliably, all existing calendar events—timed, full-day and multi-day—must be included when calculating availability. Until this is implemented, the booking feature should ideally be marked as beta or temporarily withdrawn.

I didn't even realise all day appointments don't show as unavailable, this is essential to avoid booking conflicts! Also otherwise adding long holidays to the booking calendar is such a hassle

Bookingpage is useless for me without this.

This is critical to consistently produce correct bookings!