> I do not think there is any way they can legally request us to provide that info
What makes you think that? National id numbers are not regarded sensitive personal information (source 1 and 2) and you just provide it so that AirBnb can comply with regulations set forth by the Norwegian Tax Authority . Furthermore, you are using AirBnb's service at their terms, so as long as they are within the boundaries of the law, they can ask you to provide whatever information they require to comply with the tax regulations. They cannot force you, of course, but as it is their service and their rules they can kick you out, as they also specify:
It is important that either your national identification number, D-number or organization number are provided, along with a complete listing address to satisfy this tax regulation. Failure to do so will result in your Airbnb account being suspended.
You are totally right about the tax returns, of course, as has been the rule up until now, but the changes imply that you will no longer need to supply this, as it will automatically be submitted by the rental agent 🙂