Comment # 1 on bug 1145007 from Jan Kara

This is a result of commit 9e8925b67a80:

    locks: Allow disabling mandatory locking at compile time

    Mandatory locking appears to be almost unused and buggy and there
    appears no real interest in doing anything with it.  Since effectively
    no one uses the code and since the code is buggy let's allow it to be
    disabled at compile time.  I would just suggest removing the code but
    undoubtedly that will break some piece of userspace code somewhere.

    For the distributions that don't care about this piece of code
    this gives a nice starting point to make mandatory locking go away.

So I suppose not supporting mandatory locking anymore was a deliberate
decision. This is actually nothing ext4 specific - rather a generic VFS change.

Do you really use mandatory locking or was that just a leftover from the past?