A) Very. If I’ve made plans with someone, and they don’t confirm or respond to my attempts to confirm, I consider those plans canceled or placed on hold.
B) It’s a courtesy, but not a dealbreaker.
C) If I make plans with someone, I just assume they’re on unless I’m otherwise notified. Confirmation is unnecessary.
*Confirmation means, for example, a text exchange of “Still on for 6pm?” followed by a thumbs-up emoji.