My original thought is:
Last say a post is a year old is edited today. Shows as edited with today’s date.
Then this post is worthless for ban evidence correct?
So scumbags can simply use all edited posts to copy and paste?
Or am I missing something?
If it's already been edited at the time when the spammer copies it then the edit timestamp will still be earlier that the spammer's post timestamp. For this to work the spammer would have to see future, i.e. that a post they copy will be edited after they copy it, or they would have to copy their own posts from alt accounts and then go back to edit them.
So I don't think it would be easier than e.g. copying something from some obscure site that's not indexed by Google, or using one of those word-spinning tools. Most spammers are extremely lazy and spam for pennies so they won't be putting that much effort into it.
Okay this solves my question.