We're talking about Git here. The question is not "how VSCode is programmed", the question is "does Git have a special field for commit trailers". The answer is no. Git stores the trailer as part of the commit message.

If you look at the comment I’m responding to, it is in fact about how VSCode is programmed; specifically, a possible reason why the Co-Authored-By trailer doesn’t show up in VSCode’s commit message box.

It seems like it would be most reasonable to consider porcelain vs. plumbing command details in deciding if something is logically distinct to Git. git-commit has --message and --trailer options, git-commit-tree has a --message option. I take that as trailer is a convenience option to provide a consistent way to append those details to the commit message. But that doesn't mean it's not part of the commit message, nor that the user shouldn't see it while reviewing the commit message.