Situation:

If users have too many branches in their repo, they can maintain their repository by deleting unused branches.

The next step is to close/decline "Open" pull requests with those deleted branches as source branch.

Workaround:

The only way to do this is by manually declining pull request in Bitbucket UI or identify all pull requests with the deleted source branch then use the API endpoint below.

/2.0/repositories/{username}/{repo_slug}/pullrequests/{pull_request_id}/decline

https://developer.atlassian.com/bitbucket/api/2/reference/resource/repositories/%7Busername%7D/%7Brepo_slug%7D/pullrequests/%7Bpull_request_id%7D/decline

Feature request

It would be great if we can decline/close open pull requests in bulk in this kind of situation.
Or probably add/update an API endpoint to filter pull requests with a deleted source branch