The SEDE help page says (emphasis added):
…an open source tool for running arbitrary queries against public data from the Stack Exchange network. Features include collaborative query editing for all graduated and public beta Stack Exchange sites.
And the SEDE schema FAQ post says (emphasis added):
Stack Exchange releases "data dumps" of all its publicly available content roughly every three months via archive.org, and also makes that information queryable over the Internet at the Stack Exchange Data Explorer (SEDE) (updated weekly, on Sunday).
I've seen several posts on MSE that also seem to indicate that the basic logic is something like: "If that info is already available to the general public through the Stack Exchange network sites, then it can go in SEDE, and if not, it should not go in SEDE". (Where "general public" means anyone visiting the network sites with or without an account, and regardless of reputation level).
Feature-request posts that seem to support this:
is:question [data-explorer] [feature-request] public
Can some metadata about deleted posts be included in Data.SE?, where some metadata about deleted posts got added to SEDE (despite it not being otherwise available to the general public), but also that some specific metadata did not get added to play things safe and avoid releasing info and possibly regretting it later for currently unknown reasons:
That doesn't mean we can't add more fields in later. For the moment, we just want to be sure that we not releasing a djinn who refuses to get back in the jar. - @John Ericson
Should we include information on whether post activity was from one of the mobile apps in the public data?:
@TimStone: Not necessarily a permanent thing, but I can't give out the queries for the mobile-related hats since they don't work on the public data. Essentially, we have a "default public" approach to information: unless there's some good reason to keep it private, we want to share what we know. User privacy is one reason for not sharing information. Hence the question. ;-) - @John Ericson
Why does the SEDE ReviewTaskResults table not include user data, which is all public anyway?:
We've taken a very cautious approach to releasing potentially private data. When it comes to SEDE, I'm always a little worried that people will create queries to target specific users for bad behavior. It's one thing to have the data public, but not easily searchable and quite another to have the data collected in one convenient location. That said, I think this is a good candidate for including in the public data and I'll see about adding it. - @John Ericson
Why is chat.stackexchange.com not exposed in Data Explorer?, where the top answer says (not really authoritatively):
I guess the argument can be made that chat transcripts shouldn't be made public through the data explorer. Different from the Q&A data, that's not really in the "public interest". - @Pekka
I'm not really sure if I've gotten this right. It'd be great if someone with more authority / experience on the subject could peer-review this post.