Yes. For us these are exceptions rather than drivers of software delivery. External events are certainly hard dependencies, but more often than not, the deliverables are more marketing than software. That is, a proof of concept, something behind a feature flag for the event - that will not be used in production. We do this routinely, by placing a PoC on top of the backlog. Certainly not for all organizations , but I have had the fortune to shape this model company-wide.