I disagree that this belongs in initramfs-tools. Initramfs-tools should not be responsible for mounting every deprecated filesystem that some other package providing initramfs hooks requires. This is mdadm's problem to fix, NOT initramfs-tools (and then mdadm should stop mounting it as soon as it is fixed to use the not-deprecated firmware interface).
I disagree that this belongs in initramfs-tools. Initramfs-tools should not be responsible for mounting every deprecated filesystem that some other package providing initramfs hooks requires. This is mdadm's problem to fix, NOT initramfs-tools (and then mdadm should stop mounting it as soon as it is fixed to use the not-deprecated firmware interface).