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,
Then, this will be an advantage to plagerists who post in Mega threads where their posts can be easily buried and maybe they may never be detected.
they did not just come a number of months there is a mathematical calculation behind these six months.
I don't think that theymos did any special maths in this case. The only maths is that six months is a good time to catch a spammer, any spammer not caught within 6 months should escape. Besides there's always a warning when one wants to post in a thread that is more than 120days old.
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.
I don't understand what you mean by recalling a deleted post. Do you mean going to inbox to repost same deleted post (maybe editing it or not)?