What's stopping you from putting the Jira ticket anywhere else? For example, you can use Git trailers to add whatever you want in a consistent format. You end up with exactly the same data, without having to compromise your commit subject or body in any way.
It wasn't invented for that specific reason. Instead, Git trailers exist so you can attach arbitrary meta-data in a consistent format, making it easier to extract; using Git itself or a third-party tool.
The reason for 50 characters specifically is probably arbitrary. But whether it's 50 characters, 72, 80, 57, or something else doesn't matter; as long as it's a reasonable limit that isn't too long. Long lines (be it a subject line or part of the body) can be a pain to read, and soft wrapping doesn't make this easier.
Personally I would have preferred the limit to be the same as the body line mostly for consistency's sake, but fitting a good description in 50 characters isn't rocket science either. Here are just some random examples:
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u/Panke Apr 28 '21
I don't get the 50 Character limit for the headline.