As already noted, we're excited to have shipped the first iteration of work in this area earlier this year in v7.9, available for those running Bitbucket Data Center in a clustered environment.
This allows you to upgrade to our security patches and bugfix versions on minor releases with no downtime for end users. E.g from 7.9.0 -> 7.9.1. We know that for those of you who stay on versions of Bitbucket for longer periods, particularly LTS releases, this will allow you to easily keep patched without downtime.
You can read more about it in the release notes here and see an overview of an end to end upgrade experience on our Data Center Video Library page here.
We're not stopping here though. We're continuing to investigate how we can eliminate further downtime and look forward to giving more concrete updates on this in the near future.
As always, if you'd like to be involved in feedback sessions, we'd love to speak to you. Please reach out at rsaunders at atlassian dot com
Product Manager - Data Center