Summary:
When connecting to a Bitbucket Server older than 5.2.0, the Blue Ocean pipeline creator claims that the Bitbucket server is "too old to work with Jenkins." This message isn't accurate. Jenkins works fine with the same BBS elsewhere. It's only the New Pipeline flow which doesn't work.

Prerequisites:
A functional Bitbucket server which is running an old version of Bitbucket. In my case, it is running 4.14.4.

Steps to recreate:
1. Go to Manage Jenkins, and set up your Bitbucket Server with the correct settings. It's also possible to demonstrate this problem by supplying the Bitbucket Server URL via Blue Ocean:

2. Launch Blue Ocean. In the below screenshot, I already have a Pipeline created on my Bitbucket server.

3. Click "New Pipeline"

4. Select the Bitbucket Server that you set up in Step 1 above:

5. Note that Blue Ocean responds by telling you that "This Bitbucket Server is too old (4.14.4) to work with Jenkins. Please upgrade Bitbucket to 5.2.0 or later."

6. We know that this is not true, becuase it's already been configured from Classic. And there's an existing repo, with a Jenkinsfile, which is successfully building.

7. Bringing this back full circle, one can at least load the preexisting Jenkinsfile into the editor, and attempt to make changes, without any mention of an incompatibility between old versions of Bitbucket Server and Jenkins. I believe there to be some other, more serious problems involved in trying to edit these pipelines, but I'll save those for their own JIRA issues.