Here's the explanation of the "optional" email that went out today:
ERAS created this supplemental application. As you all know, it's an experiment -- the idea is to test new application ideas on a smaller pool of applicants / programs, and then things that are useful will be added to the standard app. Two of those new ideas are signaling, and geographic preference. For Geo preference, applicants will be able to choose up to 3 regions, or will choose no preference. Signaling has been discussed before.
When programs had the ability to sign up, we were asked to choose whether the supplemental application would be optional or required. Presumably that information would be visible to you as applicants.
Well, the NRMP reviewed all of this. I would have assumed that ERAS would have reviewed this idea with the NRMP -- perhaps they did and the NRMP was just slow to make decisions. In any case, the NRMP decided that making the supplemental app required, and then asking for geo preferences, violated the terms of the NRMP agreement for programs. Remember that programs are not supposed to ask applicants where else they have applied or are interviewing. Apparently the NRMP decided that asking for Geo Preferences was similar enough to asking where else you applied.
So, the solution was to make the supplemental app "optional" for all programs. We can't force you to complete it. Which is somewhat strange, since you could choose "no preference" (even if you had a preference) and hence this whole issue was optional anyway.
But I don't think this changes things for applicants. Even though every program will be listed as "optional", it's quite possible that programs will focus on those applicants who completed it. But some programs may not, and consider apps without the supplemental equally. I still don't know how it will affect our decisions -- can't really know until we see it, see how useful (or not) it is.