From what i've heard the "explored" area is going to be 500 meters past the edges of your revealed map. Which is actually a pretty long distance when you realize the default reveal while exploring is only 50 meters or so. What this means is the terrain in those areas has already been generated.

That said aany areas of the map marked as explored will not be updated with new terrain as it is added. So if you have explored a lot of your worlds map then most of that world should be considered "exlored" and you might want to think about creating a new world when the next big patch comes out.

If however you are OK with using mods once in awhile you have an option. The Upgrade_Worlds mod available at

https://www.nexusmods.com/valheim/mods/1485
and
https://valheim.thunderstore.io/package/JereKuusela/Upgrade_World/

lets you reset portions of your world, marking it as "unexplored" in the games world file so the next time you enter those areas they will be regenerated using the latest terrain generators. You only need to run the mod once then you can disable it until needed later.

The mod will let you define areas of the map that will not be reset so you can protect any structures or favored areas from being reset. Keep in mind this will completely remove everything in the unprotected areas so be sure to back up your world and use with caution.

I hope this information is of use to you.