This is one of those royalty free backgrounds you can buy from Digiket/Booth that let you use them as they are or edit them and use them in your products. You can find the pack here: https://booth.pm/ja/items/3759551
We’ve been trying to replace various backgrounds with original art, but they take time and sometimes the artists aren’t available, so we have to use assets like this. The downside is since it’s available for anyone to buy, we might run into a situation like this. This pack is pretty common for medieval/fantasy setting gamed, so you’ll probably find other games using it as well.
- Punipen