Questions should no longer be closed just for not having been sandboxed.
Now that the "hasn't been sandboxed" close reason has been deactivated, riddles are allowed to be posted on the main site without having been through the sandbox first. Leaving a big notice on several questions saying that they aren't is misleading and likely to confuse new users. The question you link to currently has a banner on it which says:
"Certain questions cannot be posted on Puzzling Stack Exchange unless they are sandboxed first. This will last until more complete quality standards are developed. For more information, see this meta post." – GentlePurpleRain ♦
Seeing that on a question, signed by a mod, could be really confusing to people, especially if they've also seen new non-sandboxed riddles being posted. All questions closed for this reason should be reopened as quickly as possible, using a modhammer if necessary.1
(Of course, this doesn't at all mean that riddles shouldn't be closed any more - they just shouldn't be closed for this particular reason. It's quite possible that some of those closed for the "hasn't been sandboxed" reason should be reopened and immediately closed again as "too broad" or whatever.)
I used the moderation tools
(viewable by 10k users only) to find out how many questions we're looking for to reopen:
So there are seven questions currently closed for the "hasn't been sandboxed" reason. Unfortunately I haven't been able to find them all; the only ones I've found other than the one you link to have been automatically deleted by the system due to having been closed and inactive for a while:
Whether these should be undeleted and reopened is arguable. My argument above of confusing new users no longer applies, since there are only 17 people on the site who can see deleted posts. Since both of them have received negative feedback in the sandbox, I'd be in favour of leaving them as they are and encouraging their OPs to repost if they can sufficiently improve the riddles in the sandbox. (That's assuming the OPs haven't been automatically question-blocked due to having a question closed and deleted, of course; if this is the case, those riddles probably should be undeleted and reopened for fairness's sake.)
1 Can it still be called a modhammer when it's being used to open things rather than close them? Hmm ... I dunno. A mod crowbar maybe?