I'm one of the Product Managers at MongoDB and I was a part of making this decision. It seems like there are two main areas where people are asking for more information: 1) exactly what are the numbers of Solaris downloads and 2) what about security patches of existing versions? I'm happy to clarify those points here and we will be posting an update to the blog to include this information.
Based on the data we have, about 0.06% (and decreasing) of MongoDB users are running on Solaris. In addition, as Andrew mentioned in the blog, all of our customers have either deprecated Solaris or have told us they plan to deprecate it.
In regards to security patches on Solaris, we will continue to fix critical flaws for the community, regardless of where found or how reported. Anyone can report a security vulnerability by using our Security project (https://jira.mongodb.org/browse/SECURITY) to create an account then a ticket describing the vulnerability.