When positively-scoring posts are deleted like this and the user account is also deleted (anonymised to a grey "user12345" with no link), it generally means the user has been destroyed - their account deleted and all their posts with it. There may be several different reasons for this - which are private between the mods and the user in question - but a common one is that the user was circumventing a suspension on their main account and the destroyed account was a sockpuppet.
There's a policy
on main meta that reasonable questions posted by a troll user shouldn't suffer for it. This comes under the overall "vote for the post, not the person" policy on SE:
If it's a legitimate question, it shouldn't matter who asked it. Don't worry about who's asking the question, judge the question on its own merits.
As such, if the post isn't spam, I say it shouldn't be flagged as spam. If it's a good question, it probably shouldn't be downvoted (Of course, your votes are yours to do with as you will). As others have stated, however, flagging the post for moderator attention to let them know that the troll is circumventing post bans is perfectly legitimate. I just don't believe that a good post from a bad user should suffer for the actions of the asker.
So if you come across a good or reasonable question which has been deleted for no other reason than that the OP was a troll or evading a ban, you should flag it for mod attention and ask for it to be undeleted. Furthermore, I suggest that the mods here follow the same approach to deal with such users as was used by the SO mods in this case (emphasis mine):
I've dealt with many of this user's accounts, and tried to figure out a way to handle them, so I can explain my approach so far. First, I should comment that this user has been an ongoing nuisance since the start of the year, and we've probably deleted something like 100+ of their accounts since then. They've worked around IP bans and other extreme measures to continue to post here, and have posted some rather odd content. [...]
However, I leave any good questions alone. I do not destroy their accounts (which automatically removes everything posted by a user) and instead simply delete them. Unfortunately, even the latter has the consequence of triggering a deletion of downvoted content by that user, so sometimes acceptable but downvoted questions get removed at that point.
As far as I know (speaking as someone who has experience with SE mod tools from a different site), there's no particular advantage of destroying over deleting when dealing with sockpuppets circumventing suspensions. Deletion using the "no longer welcome" option has most of the same side-effects in terms of hobbling the user as destruction, and doesn't require removing good content.