Yes, I have seen, it thanks—a couple of years ago from memory (noted that, unhelpfully, posts on the forum do not have a time stamp). It's no more use now than it was then. I mean, what's the use of a "hidden file in bookmarks"? Why hidden? This is about as helpful as a parachute that opens after the first bounce.
If indeed you believe pinboards are stored on the user's device, as you appear to suggest, please tell us where.
Oh, also noted are the references to using Sync as a form of backup. Great idea, if only sync would work, but it refuses to. I posted about that a couple of months ago but no-one bothered to respond.
I wonder how many users have given up, exasperated with Opera because the marvellous features it offers, eg. pinboards and sync, are buggy, fragile, and time-wasting when the you have to spend hours trying to work around the flaws, to say nothing of the hours spent setting up pinboards only to have them wasted when they are arbitrarily deleted for no reason. These issues should have been dealt with years ago.