I'd say 6 months is great. Is there any reason to report an old post when we have a ton of current spam reports still unhandled? Old posts will only be moderated if they fail to meet the criteria mentioned on the report page, an old topic without a single or few spam replies is likely to get moderated .
With this recent implementation, does it also mean that any plagiarism that isn't detected with the space of 6 months can go unpunished?
That may well be the case, the positive side is there will be no backlogs, I think the 6 months are calculated to ease the backlogs and concentrate on the new incoming spam posts, they did not just come a number of months there is a mathematical calculation behind these six months.
And besides it's very surprising to get a notification that your posts back 4 years ago were deleted you will even be surprised by your posts you can't even recall some of it, but getting notifications that you have posts deleted within the six months period you can recall your posts and you can improve your posts to avoid that kind of posts.