[teilo]

Not true. Immutable works just fine on ext4.

https://pastebin.com/beuimxR8

[jwilk]

Indeed. The list of all supported attributes is in the manual page:

http://man7.org/linux/man-pages/man5/ext4.5.html#FILE_ATTRIB...

[reptation]

Ah, you're right, I mis-remembered: """ The `c', 's', and `u' attributes are not honored by the ext2, ext3, and ext4 filesystems as implemented in the current mainline Linux kernels. """