The error comes from bitbucket server which means we cannot issue a warning. Instead we would have to enhance Drone with the following:
Drone would need to allow you to configure the version of Bitbucket server when you setup your Drone server (e.g. DRONE_STASH_VERSION=5.x.x) and;
Drone would need to gracefully downgrade API requests to work with older versions of Bitbucket server. Specifically, if you are using Bitbucket Server 6.x or below, Drone would omit pr:from_ref_updated when creating webhooks.
For now, your best option is to run Drone version 1.8 in production, since this change will take some time to design and implement. We will add this to our roadmap and update this thread once we have progress to share.