Well sort of.

If you started your current world prior to the Mistlands update you will still find the Mistlands but...

The content drop that added the Mistlands also changed the land building algorithm a bit so to create larger Mistland land masses.

So you will be able to find the Mistlands in your current world, but they may be more a series of broken, small islands than the larger land masses you would get starting a world post content drop.

And don't forget, with just a small effort you could transfer all your high value items to a new world with your character if you so choose, so your past effort would not be lost.