This mod will show you all explored areas of your world, by rendering them when you go near them, and saving the pictures. It's not perfect quality, or even good, but it works surprisingly well.
Note that it can use quite a lot of disk space - hundreds of megabytes in my testing, probably gigabytes for larger worlds.
Consider this an alpha release. Current limitations and issues:
(Original mod by immibis. Original description from the FTB Forum)
This mod is abandoned and has been uploaded for archival purposes. It was originally created by immibis. It has been uploaded to Modrinth under the ModrinthArchives account with permission from the original author.
!Discord screenshot showing permission to upload