Baloo

Baloo replaced Nepomuk, see https://community.kde.org/Baloo .

Baloo and extended attributes

Applications such as Dolphin store tags and ratings in xattrs (extended attributes) in the filesystem, and Baloo indexes these so you can search them. On Linux, to show them as entered in Dolphin, in a terminal enter: getfattr /path/to/file assuming the necessary package ("xattr" in Debian) is installed.

Some filesystems (e.g. FAT32, the filesystem which which most USB flash drives are formatted) do not support xattrs at all, so copying files to such a volume will lose the tags.

Most CLI utilities (e.g. cp, tar, rsync) won't copy xattrs without additional flags either, so unless your environment is 'All KDE all the time' and you only use Linux filesystems like ext4, XFS or Btrfs (this is not a complete list), and you modify your backups to specifically include xattrs, your tags may get trashed one way or another.

Nepomuk (the previous KDE tagging/file metadata system) used to store tags in a separate database, but even after many years of development it could hardly be made to work performantly or reliably so it was replaced with Baloo.

In some ways using xattrs is 'the right place' to put this tagging info, but Linux doesnt have a 'core API' that would allow a 'xattrs preserved by default, and interpreted consistently' policy. Therefore there are a lot of ways to lose your tags. KDE has been trying to make this work for many years, but it is difficult given the way UNIX/POSIX has evolved.