If you do these things:

* File issues in a project tracker (Github, jira, asana, etc)

* Use the issue id at the start of every commit message for that issue

* Use a single branch per issue, whose name also starts with the issue id

* Use a single PR to merge that branch and close the issue

* Don't squash merge PRs

You can use `git blame` to get the why.

git blame, gives you the change set and the commit message. Use the issue id in commit message to get to the issue. Issue description and comments provide a part of the story.

Use the issue id, to track the branch and PR. The PR comments give you the rest of the story.