Comment by grishka

Those "nandroid" backups weren't "close" to that, they were literally that initially. Then, when Android phones started coming with the /sdcard partition mapped to the internal flash memory (a subdirectory of /data) instead of an actual SD card, the /data partition backup mechanism was changed to copy individual files into some sort of archive, but the end result remained the same.

You can still do it on modern Android devices, as long as the bootloader is unlocked. Yes, the file system is encrypted, but a modern custom recovery is able decrypt and mount it.