How long do you need to keep plan records?
Remember that employee that got divorced 15 years ago. The Ex got a QDRO and you paid them out and they are history right? Wrong!
They forgot they got paid and as they approach retirement they come back saying they know they are owed some money. They may even have a statement showing they were due some money back then.
Now what? If you have copies of the checks, you are home free! And maybe even if you have copies of the 1099's. But if you have no records you are likely to be in litigation and paying out that benefit a second time!
So when it comes to distributions, you need to keep those records for a very long time.