In the past, for new hosts who were worried about maintaining their Response Rate,
experienced hosts told these new hosts that a host did not need to either Pre-Approve or Decline an simple Inquiry (as opposed to a Reservation Request).
In the past merely sending any sort of message back to the guest was enough to fulfill your obligation regarding Inquiries.
This has now changed.
Sometime recently, AirBnb modified its policies so now hosts DO have to either Pre-Approve or Decline Inquiries to maintin their Response Rate.
The new policy is outlined here:
https://www.airbnb.com/help/article/430/what-is-response-rate-and-how-is-it-calculated
I have contacted Host support to specifically ask about the policy change, and they did verify that the currently active policy is different than it was in the past.
So the new advice is this: hosts DO have to either Pre-Approve or Decline Inquiries within 24 hours to maintain their Response Rate.
(Just like they have to with actual Booking Requests.)