After taking a closer look at logs and source code I wish to add the following:
1) I'm only seeing the "Unable to update the mtab file" warning after messages of the form ">> umount.nfs4: <mountpoint>: device is busy". It turns out that umount.nfs4's EX_FILEIO return code, which autofs calls MTAB_NOTUPDATED, is really more generic: in my case it's returned because umount2(2) set errno to EBUSY, and umount.nfs4 didn't even call update_mtab(). In other words, arranging for automount to pass -n will (confusingly) not suppress the warning in this case. Looking at nfs-utils' support/include/xcommon.h I get the impression that umount.nfs4 ought to return EX_FAIL rather than EX_FILEIO, but I'm worried that changing this may break other programs that rely on the current behaviour. Anyway, this is for the nfs-utils, not the autofs, maintainers. 2) /etc/mtab has been a relative symlink since systemd v215 (released in 2014). Adding ../proc/self/mounts to the list of symlink targets that cause automount to pass -n to mount/umount would help all systemd-based distributions, not just Debian. It's a rather minor bug, though. If nfs-utils won't or can't change the return code there might be a case for rewording the warning emitted by automount and/or downgrading it to an informational/debug message.
