I am running into the same issue with the
rabbitmq_message_deduplication plugin on Kubernetes. Same error.
init terminating in do_boot ({error,{error,{amqp_error,command_invalid,unknown exchange type 'x-message-deduplication',none}}})
Running the latest rabbitmq-ha helm chart (using the RabbitMQ 3.8.5-alpine image) and setting definitions in values, which creates a definitions file in the right dir in the rabbitmq container, so the process is a bit different than what OP describes, but the result is exactly the same.
It appears that when the definitions are loaded, the plugins involved aren't registered yet, so the exchange type does not yet exist, causing this error. This may be an issue with the order in which things are initialized, or it may be an issue with how the plugin registers its exchange types. If someone from RMQ can confirm which one it is, I can open a ticket on the correct repo.