chattr (Change Attribute) is a command line Linux utility that is used to set/unset certain attributes to a file in Linux system to secure accidental deletion or modification of important files and folders, even though you are logged in as a root user.
In Linux native filesystems i.e. ext2, ext3, ext4, btrfs, etc. supports all the flags, though all the flags won’t support to all non-native FS. One cannot delete or modify file/folder once attributes are sets with chattr command, even though one have full permissions on it.
Full article here:
5 ‘chattr’ Commands to Make Important Files IMMUTABLE (Unchangeable) in Linux (Tecmint)