Paragraph as written:
vfs.zfs.write_to_degraded - Control whether new data is written to a vdev that is in the DEGRADED state. Defaults to 0, preventing writes to any top level vdev that is in a degraded state. The administrator may with to allow writing to degraded vdevs to prevent the amount of free space across the vdevs from becoming unbalanced, which will reduce read and write performance. This value can be adjusted at any time with sysctl(8).
The third sentence has very awkward wording:
The administrator may with to allow writing to degraded vdevs...
A commit references this bug:
Date: Wed Oct 1 02:46:59 UTC 2014
New revision: 45701
Submitted by: firstname.lastname@example.org