It depends on how good your water quality management is during the winter.
If you can maintain good KH, GH and oxygen levels, plus water circulation, you'll be fine.
It will have the same effect of throwing leaves in your pond just before winter so the oxygen level will be reduced quicker than without adding any organic material.
Organic material decomposes during the winter but at a slower rate.
Sounds like I should wait until spring to put the barley straw in since we go to Florida for a couple of months and I just have two aerators for water movement during the winter months. Thanks for your input Mitch
Don't bother with barley straw. It has never made any difference in my pond. At best, it's a band aid for the pond. In my case, it was a band aid to cover bad pond water chemistry management. Once I increased water circulation, added more media surface area, I never needed to use it again.
Studies have shown that humic substances which are leached from organic material such as leaves and barley straw have anti-algal and antibacterial properties for the pond water and it's inhabitants.
Perhaps sometimes people have unrealistic expectations.