/etc/default/grub.d/dmraid2mdadm.cfg is in mdadm 3.2.5-5ubuntu4.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1
GRUB_CMDLINE_LINUX_DEFAULT="$GRUB_CMDLINE_LINUX_DEFAULT nomdmonddf nomdmonisw"