Here's my experience:
In Shanghai (PVG), I went to the counter to check in, the guy barely spoke English, asked me what was in the bag. Eventually he checked me into the flight and attached the luggage barcode thing to my bag. Then he called someone on the phone and told me to wait for someone to collect the luggage. A few seconds later he asked me to take it to another counter a few rows down. I lined up and eventually gave the lady the bag, she mentioned it was oversized and put it behind the counter.
In retrospect I should have asked them to take it to the oversized luggage counter at the airport myself as when I arrived in Tokyo they couldn't locate my bag (they found it a day later and shipped it to me) so I'm guessing it found its way to Tokyo on another later flight after someone forgot to take it to the oversized luggage counter.
In Tokyo (NRT) they said that the bag was over the limit of 200cm after measuring the dimensions of the bag. But after I explained that it was a standard sized snowboard bag and that I took the inbound flight here without problems, the lady looked up her computer and said OK her mistake. Someone escorted me to the oversized luggage counter to check in my bag and this time I was confident it would make it to Shanghai.
In summary, I don't think the checkin counter staff really know the rules and what to do with a snowboard bag, and I was a little lucky not to get charged a fee.
After some research I found this page by The Inertia, which lists many airlines' policy towards taking snowboard bags onboard
In principle, Air China do accept snowsports equipment, subject to max
linear dimensions of 158cms - which excludes every adult ski on the
market. However, when we called Air China, we were told that you can
take a longer ski bag (up to 23kg) for '33% of the standard rate,' or
approximately £15 if flying from the UK. In short, call them when
booking to confirm fees for your flight!