It's not about mutability of events, but about mutating the underlying data itself. If the event only says "customer 123 has been updated", and a consumer of that event goes back to the source of the event to query the full state of that customer 123, it may have been updated again (or even deleted) since the event was emitted. Depending on the use case, this may or may not be a problem. If the consumer is only interested in the current state of the data, this typically is acceptable, but if it is needed in the complete history of changes, it is not.