Ok, I tested this myself because I couldn't find any information elsewhere on this but it APPEARS THAT:

If you're using an OLD world and heading out to find mistlands in unexplored area, which is what the devs said to do; YOU WILL get Mistlands, but with the caveat that the Mistlands you -DO- get are very broken, small islands, hard to navigate and pretty much unplayable.

I generated the same seed after the mistlands patch and the mistlands I went to were completely different, actual land masses and navigatable terrain. It seems like currently you need to generate a NEW seed to play this biome.

tldr: yes, you'll get it in old seeds in unexplored biomes, but it'll be broken and hard to play. You need to generate a new seed to get the intended experience.