No you can't. No one has ever been relegated to D6.
And just in case you're the sort of enterprising fellow or lass who wants to see these truths with the very own eyes in your head, look at your league standings and then switch it back to season 19. Look at the team that finished 8th, and in their team history it will say that they were relegated from your league, but they are still there. (There may or may not be a new manager controlling them and if they got a new manager the team's been renamed most likely, but it's the same franchise.)
Nobody will relegate to VI in the USA unless and until there are enough managers to fill up all five league levels first. If we were to top 5456 active users, they'd have to open up VI to accommodate any more users. But there's unfortunately little chance of that coming to pass any time soon.