I want to setup build status notifications for a Microsoft Team channel but it does not appear to be natively supported in the declarative pipelines and there is no UI option for Multibranch Pipeline.

Suspect the best place to support to set the webhook URL and requires state changes in the options section of the declarative pipeline: https://jenkins.io/doc/book/pipeline/syntax/#options

office365ConnectorSend isn't really a solution for this, as how would we capture when a build fails/succeeds after previous failure/etc....