I was working in Portland in 2016 when Inclusionary Zoning was instituted, for a company that designed a lot of medium-sized apartment buildings. We witnessed the rush to submit permit applications for 20+ unit buildings before the new requirements hit, as well as the nearly instantaneous shift to 19-unit projects, including splitting parcels into smaller lots and buildings.
I'm not opposed to smaller-scale density in general, but this article does a good job of laying out the ways that developers have avoided IZ requirements in the intervening years and the resultant housing shortage which is antithetical to the supposed intent of Inclusionary Zoning.
Obviously, we need creative solutions to affordable housing. We addressed some of these ideas in our recent post on removing barriers to affordable housing, and there are countless other innovations out there. However, growing evidence suggests that Inclusionary Zoning is not the silver bullet it was intended to be.