Dragon Quest XI S allows players to enjoy the game in both 2D and 3D, and it's an option you can switch between as you play. Here's a word of warning, though. If you're switching between the two modes a bit too often, you could end up costing yourself some progress. You have to be careful with how and when you swap between the two modes, and then you should be fine. Reddit user Passerbytiredoldman has given a detailed explanation on the situation, which you can read below.
When switching from 3D to 2D (or vice versa), you don’t continue where you immediately left off; it seems like you choose a “chapter” to start from. No matter what chapter you start though, all of your levels, items, and equipment carry over.
To unlock “chapters” to choose, you have complete it. The new chapters seem to show up when you finish the main quest in a certain town/chapter and are on the way to the next town/chapter.
Now the big warning – If you pick a prior chapter than the current one you’re on, it resets the entire story to that chapter. So as an example, you’re on chapter 4 of the story and you were curious about the 2D mode, so you pick the very beginning. Your levels and inventory carry back with you, but this seems to “reset” the entire story back to the beginning, so if you accidentally did this on your main save file (like me), you have to play through the story all the way again. When you try to make the switch back to 3D, the only chapter you’ll have available is the beginning and you need to unlock the chapters again by progressing through the story.