Currently, when integrating with Jira, Links (repository settings > Links) to that instance will automatically be added to all repositories on the linked workspace.
On the links page, it's possible to add a restriction to require issue keys in all commit messages, and those issue keys need to match an existing issue on the linked Jira instance.

However, currently, it's not possible for users to configure Bitbucket to only accept pushes with commit messages that have specific issue keys, Bitbucket will allow users to push as long as the commit message matches any issue key in Jira.

The request here is to add the ability for users to configure which issue keys should be required on the commit messages for each repository.