No. I don't use backorders.
To my knowledge, based on my own experience as well as that of other domainers who have attempted this, it's not possible to win GD Auctions domain by placing a backorder post-auction and pre-closeout. I've never talked to anyone who actually go this to work, though many have tried. If this was possible everyone would be doing this rather than trying to "snipe" domains as closeouts.
Has anyone here successfully placed a backorder after an auction has ended and won a GD auctions domain in this way? With a zero percent success rate I haven't tried this for some time as it's just a waste of time, so I wouldn't be aware if it has recently started working. If you ask GoDaddy support they will
say that it works, but GoDaddy support is often clueless about how their platform works, especially auctions, and there is nothing from my own and others first hand experience that indicates that it actually works.
The only way to get a backorder to work on an auction name, to my knowledge, is to place the backorder while the auction is still going on, which will then show up as a $10 bid in the auction (and extend the auction if you place it last moment when there is 1-10 seconds left). That's practically the same as placing a $12 bid, only difference is that you pay slightly less. In terms of visibility/not attracting competition there are no benefits to doing so.
There is one party at GoDaddy auctions that on some days place backorders on thousands of domains while they are still in auction. These backorders are placed around 5 minutes before the end of the auction. As a result, they "open" bidding on all of these names, and those that are decent will get other bids, while they get the garbage ones for their $10 opening bid. What happens in this scenario is that human domainers will cherrypick/bid on the best of these domains after bidding has started, while leaving the scraps to be won by these automated $10 backorders, unless "bidder 1" enters the fray and bids too (which they do at times, though seemingly not most of the time). Overall I think this is a very inefficient approach, as by opening bidding you bring attention to these names, and as a result other domainers will only let you have the worthless ones for $10 as they will bid up those with value. From what I have seen, both HugeDomains and another party that uses WHOIS privacy have used this method of placing a backorder while the auction is still live.
A far more effective approach for them (if it was possible) would be to delay the backorders by 5 minutes, let the domain auctions just end, and then place the backorder a second after auction end, and get them before they reach closeouts. That way they would be able to win far more domains, and of better quality, than they currently do by placing backorders 5 minutes before auction end. I think the fact that they place backorders 5 minutes before auction end, rather than 1 second after auction end, is another indication that you can't win domains by placing a backorder post-auction pre-closeout. If the latter was possible, the strategy of placing a backorder 5 minutes before auction end would be completely counterproductive and self-defeating, and there would be no reason for anyone to place thousands of backorders on any given day before auction end.
That doesn't answer your question of what is happening to domains that don't seem to reach closeouts, but at least I doubt that the answer is that these domains are backordered after auction end.
On my end I have not seen any domains I liked with a valuation of $1300+ hit closeouts for some time. Though I see that there are about 250 .COM domains in closeouts right now with a valuation of $1300+, but in contrast, there are 165,00+ .COM domains with a valuation below $1300. It's pretty obvious that someone is buying practically all .COM domains at GoDaddy with a valuation of $1300+. I don't know how they get the domains with $1300+ valuations that end with zero bids, but I
assume that they perhaps use the GoDaddy API or other automated methods to buy these domains as soon as they become available in the system as $11 closeouts. Since domains that do reach closeouts are seriously delayed, maybe API users get the closeout availability info slightly faster than site users do, and so they can easily buy them before us regular users ever even see them? I don't have API access so I have no idea about that. But I at least I really don't think they place backorders to achieve this.
On my end I am seeing sub $1300 valuation domains go to closeouts just like before, pretty much all such domains I'm following reach closeouts, only difference is that they tend to take hours, rather than minutes, to hit the closeouts.