Show Overview on Merge Requests when Source Branch is Missing

Summary

When the source branch for a merge request is missing from the project, the Overview of the merge request is no longer visible.

We delete our source branches after merging, but we also provide a lot of detail in our merge requests describing the technical changes. We do have backing stories, but we treat the stories as business requirements and technical notes may be provided, but the conclusion and impacts from a story are included on the merge request.

Screenshots

Here's an example of a merge request where the Overview section is no longer available:

The callout here is completely understandable, and I would accept being unable to see the Commits, Pipelines, and Changes on the merge request, given that a diff is no longer feasible.

Upon editing the merge request, you can see the original source, and even preview what the Overview should look like:

However, I have had a few instances over the years where someone wanted to review the details of an older merge request, as they're doing research for upcoming work. I have to keep re-educating people that the Overview is still accessible, but I think the upfront error is discouraging people from investigating further.

Expected Results

On a merge request, the Description is always accessible on the detail page, even when the source branch no longer exists.

All other forms of content (Assignee, Reviewers, Labels, etc.) would be nice to have, but it is not the core issue.

Only the Commits, Pipelines, and Changes sections should be unavailable on a merge request when the source branch no longer exists.

Actual Results

When the source branch for a merge request is no longer available, accessing the description is technically possible, but very non-intuitive.