The idea that a functioning board should disagree in private but support the group-voted decisions in public (what you call the “code of silence”) has been standard practice in a lot of non-profit boards I’ve been involved in. In a staff-driven organization, the board is there to provide oversight and strategic guidance to the organization staff. How is publicly complaining about voted decisions going to help the staff? How is that not going to hurt the organization you have a fiduciary duty to care for?
To be clear, the Platform calls for this provision to “be reformulated with less sweeping and more conventional language”. I know you have more experience with nonprofit boards than I do, but based on my research and discussion with some people experienced specifically in FOSS nonprofits, I do not think the language used here is commonly found in board agreements. Part of the concern is that it is too vague and overbroad.
For example, suppose the OSI adopts another plank of the Platform, the long overdue adoption of a process for review of previously-approved licenses. The board agreement implies that board members cannot participate in this review other than possibly in an obstructionist way, because board members are self-barred from publicly admitting to the possibility that any past decision of the OSI board was in error.
It ought to be possible for a board member to publicly express respectful dissent as to some issue that has been decided by the board without that taking the form of “complaining”. I don’t see why the choice has to be between putting on a false show of public unanimity or resigning - and resignation over a policy disagreement itself has costs for the organization and its staff.
I refer readers to the Platform which provides a number of reasons for the call for a reformulation of this one element of the board member agreement.