Appendix I from the 2013 Part 150 Study by Landrum & Brown. A Letter Of Agreement (LOA) where the Port Of Seattle agrees to using the Third Runway primarily for overflows and bad weather. Note the very official looking signature of the Port CEO, Tay Yoshitani. Also note that it is described as an ‘informal agreement’. In several places (eg. Page #1), that the FAA reserves the right to change these rules at any time to achieve a maximum of ‘safety and efficiency’ or to ‘accommodate traffic volumes. In other words, this thing isn’t worth the paper it’s printed on.
However, when long-time residents complain that the Port lied to them about runway usage ‘in writing’, this is what they are referring to. Because it does state the Port will make a sincere effort to limit Third Runway use. And this they clearly have not done. In fact, they have done exactly the opposite, making the 3rd Runway the first choice for arrivals.