Mistlands isnt released yet... the TEST branch of Mistlands was made available using a codeword, but if you aren't in the test, your world doesn't yet have Mistlands updated in it.

The test branch also (in the patch notes) uses a new terrain generation algorithm to increase the size of Mistland continents.

So old saves will have the small islands of Mistland ground while saves post patch will have larger Mistland continents.

First: the patch is on the beta branch. Mistlands can be found on the current patch, but it's just an empty area.

Second: I've been hearing that the biome doesn't properly spawn on old worlds. It comes in as a bunch of rocks jutting out of deep water, being near impossible to navigate. Starting a fresh world is recommended.

In my old world, parts of the mistlands are empty but I've also found ancient armor, trees, keeps and a mine.

After starting a new map I can't say that I like the terrain better.

Either you didn't opt into the public test beta branch, or you came too close to those areas before and they didn't update.
I've also read that having used map reveal at some point via console commands may cause everything to flag as explored, even if hidden again, causing nothing in the future to update - but I can neither confirm or deny that myself.

In any case, as Suzaku mentioned, you may unfortunately have to explore Mistlands on another world anyway. On my 500h+ launch old world, Mistlands are mostly tiny, sheer untraversable sections which in new map generation are supposed to be just the shores.
The space they left for the biome in old worlds just isn't big enough for it to properly generate most of the time.

I believe the beta patch notes have indicated that playing a new seed is required due to changes that had to be implemented.