Hardcore world can be restored by modifying the level.dat in the world directory. The problem is that players are usually lazy to do that or don't know how to do so.

This is why this mod exissts, to easily restore the lost hardcore worlds! It also offer some customizations. The mod will modify the level.dat of the specified world to restore. In case something would go wrong, a backup is done in the same world folder.

When you die in a singleplayer hardcore world, a new button will be there: "Restore World". Depending on the settings (explained below), you'll be able to recover the world still as hardcore or as a survival.

*Just in case, the mod create a backup of the world's data, named under "level.dat_backup_restorehardcoreworld"

It has no GUI on FORGE yet to configure that ingame, but you can go to config/restorehardcoreworld-client.toml and you will be able to configure that: