Currently the bitbucket branch source plugin provides the following environment variables:

"BRANCH_NAME"
"CHANGE_ID"
"CHANGE_TARGET"
"CHANGE_BRANCH"
"CHANGE_FORK"
"CHANGE_URL"
"CHANGE_TITLE"
"CHANGE_AUTHOR"
"CHANGE_AUTHOR_DISPLAY_NAME"
"CHANGE_AUTHOR_EMAIL"
"TAG_NAME"
"TAG_TIMESTAMP"
"TAG_UNIXTIME"
"TAG_DATE"

It would be very useful if this was extended to provide any issue IDs associated with a Pull Request ("CHANGE_" in the above semantics).

BitBucket Server provides a REST API to get issue IDs associated with a PR:

/rest/jira/1.0/projects/{projectKey}/repos/{repositorySlug}/pull-requests/{pullRequestId}/issues

https://docs.atlassian.com/bitbucket-server/rest/5.3.0/bitbucket-jira-rest.html

Perhaps this could be exposed by the plugins EnvironmentContributor extension as something like "CHANGE_ISSUES" ?

This would allow for use of the various Jira plugin steps to do things like update Jira tickets with information from a pipeline build of a pull request.