PRs aren't part of the repository (if you define repository to mean part of `git`'s internal working. It's part of GitHub, which is owned by Microsoft.
Small nit, but PR description bodies might wind up as part of a commit message verbatim, depending on repo settings and the merger's personal behavior. It's an easy outcome, the merger doesn't need to copy and paste or anything, and I think it might be a default or popular setting for squash-merges.