Don’t you have an id system, like an issue tracker? That’s your source of truth for change requests.

The common advice is to never have long standing branches for WIP. But if you do, squash it to have a small number of commits, then replay it on top of the default branch. The try to understand why there’s any diff or conflict (refactor, update to the design, code removal, was not merged at all,…). As I squash merge, I always remove the PR branch from the main repo. And I keep it for about a week on my local repo (In case the PR is reverted and I needed my drafting version of it).