You are completely wrong on the SOS purpose - it had nothing to do with affordability at the time. It was to stifle development. The support for SOS and the environmental restrictions were from people that were not in the well-to-do neighborhoods. They were from older areas of south Austin, Oak Hill, and other areas that in the late 80's did not have the growth and gentrification they have today. It was Bill Bunch and Mary Arnold and Bridget Shea and others that simply wanted the City to stop development. The opposition to SOS came from those white neighborhoods as you call them - that's where the developers, builders, members of the real estate community lived. They fought SOS tooth and nail. Affordability wasn't even thought about then because Austin was still in the midst of the 80's bust.
As the project that got scuttled, if it was planned it would have been built. EV restrictions do not impact planned projects - they are grandfathered. Now if it was in the planning process and didn't meet the development requirements in the code at the time, then it would not have been built. And the environmental restrictions on the east side 20 years ago were, and in many cases still are, less restrictive than other areas of the City. 20 years ago a lot of the east side had development incentives by way of being part of the desired development zone which had such things as reduced fees, higher impervious cover allowances, smart housing possibilities, and so forth.
Lastly, Tarrytown has been built out for decades. The EV regulations have had little impact there other than for tear downs and rebuilds.