Hello @Oechslein, Christoph
This does look like a service-side filtering/blocking issue rather than an invalid OData query.
Microsoft's current Azure DevOps Analytics documentation explicitly supports $select with the v4.0-preview endpoint, including queries against Iterations.
For example, Microsoft documents the same pattern:
.../_odata/v4.0-preview/Iterations?$filter=...&$select=IterationSK
So $select=IterationSK itself is valid. More importantly, your test showing that even https://analytics.dev.azure.com/?$select produces “The request is blocked” suggests the request is being rejected before normal OData processing.
Given that this started only a few hours ago, I wouldn't modify working queries to work around it yet. Please also test $filter or $top; if $select specifically triggers the block across different organizations/networks, that would be particularly strong evidence of an Azure DevOps edge/WAF regression.
Report this to Azure DevOps Support with the timestamp, organization/region, failing URL, HTTP status/response headers, and request/correlation ID from browser Developer Tools. The fact that Microsoft's own documented $select syntax is being blocked should make the escalation fairly clear.
Reference: Microsoft - Define basic OData Analytics queries
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