Requesting an execution plan from SEDE on any query currently seems to always return the message:
The 'SecurityPolicyApplied' attribute is not declared
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Right, so Data Explorer was upgraded to SQL Server 2016 last night, and as usually happens there were additional attributes added to the execution plan XML to expose information about new features, etc.
Since we were doing schema validation on this XML with the strict schema provided with SQL Server, it complained because no one updated the schema to the newest version. This happens pretty much every time there's an upgrade, and it's a pain because there's not really any value to doing the schema validation to begin with. So now we won't, pending a pull/redeploy.