Originally posted by oNsee:

So also if i didnt discover half of the map on my old save, the Mistlands biome will be very small?

If you want to know for sure you can upload your current map files into the Valheim World Generator and check the boxes for the new Mistlands points of interest and it will show you if you have any on your map.

https://valheim-map.world/

Click the blue upload box there and follow the instructions to upload your world files from your Valheim/Worlds folder.