In version 1.1 draft 01 - Section 5.2.6 - struct virtio_blk_req definition:

Data field is defined as array of 512 Byte entries as follows:
u8 data[][512];
It makes sense for IN and OUT requests, since defined for read/write sizes that are 512 Bytes multiplication.
However, it is confusing when used for DISCARD or WRITE_ZEROES requests, since virtio_blk_discard_write_zeroes struct size is 16
+Bytes.

Is implying that DISCARD and WRITE_ZEROES requests must use data-field sizes that are 512 Bytes multiplication as well?

If yes, than the device can't tell how many virtio_blk_discard_write_zeroes structs reside within data-field. In that case -
+indication for number of virtio_blk_discard_write_zeroes structs needs to be added.