I don't know about the "git log should stand on its own". Chances are that when you finally migrate to another ticketing system and the ticket numbers in your commits "break" or become meaningless, you do not care much about that old history anyway.
And even if someone is interested in an old commit that contains a ticket number for a ticketing system that does not exist anymore, the commit itself (the code changes, not the message) tell the story as well, just not in prose.
It's pretty common with configuration or infrastructure as code that a blame brings me to a commit from 5-6 years previous, where I wish to understand why the authors at the time made a specific decision.
That is more than enough time to change ticketing system, wiki platforms, or collaborative chat services.
Fair enough, I did not think about the IaC space when I wrote the comment. And I can absolutely see how a blame could take you to commits from a few years back.
I was only thinking about the application code space where elaborate code comments are definitely more common.
And even if someone is interested in an old commit that contains a ticket number for a ticketing system that does not exist anymore, the commit itself (the code changes, not the message) tell the story as well, just not in prose.