Sorry, I can’t say for sure, my post you are referring to was from April and from before Fiverr changed the Vacation Mode to the Out of Office mode, and as they also changed the inbox and other things as well since, it might or might not be as it was then and might not be the same for all of us, even, too, since they roll out many features in batches - I have still a pretty old, by recent standards, inbox iteration, for example, since I’ve seen by now 3 inbox updates posted by people, none of which I got.
As far as I can tell, “old buyers” are able to contact me, and I can reply. What I can not tell is whether I have to reply, because up to now, I always replied to return/regular clients anyway, so I can’t say if it would affect my response rate if I wouldn’t.
The sentence you quote, the wording “new buyers” is a bit iffy. I interpret it as “people who are not buyers yet but look at your gig and would contact you”, not as “buyer who just bought a gig for the first time which is still open” lol but I could be wrong.
To my understanding, this checkbox is so that potential new buyers will not be able to contact you if you can’t or don’t want to reply until you’re back from OOO, and if you uncheck that box, potential new buyers can also contact you, in addition to “current/old buyers”, and it will count against your response rate if you won’t respond as usual, within 24 hours.
I could be mistaken, though, and if you want to be sure, it’s best to ask support.