Pull requests are not displayed when the source repository has been deleted. (BB-8106)
This issue is particularly frustrating for situation when I'd like to look back at an old PR but the owner of the repository that issued the PR has deleted it in the meantime. Since the PR was merged in, the changesets all still exist so it should be possible to reconstruct all the metadata.
Even if the changesets were gone forever, it would still be nice to see the text of the PR and any discussion that happened.
Here is an example pull request that exhibits this issue: https://bitbucket.org/yt_analysis/yt/pull-request/114/calculation-of-variance-for-1d-profiles
A number of other pull requests for yt_analysis/yt exhibit the same issue.