For what it's worth, you can have your own local gitignore by adding patterns to .git/info/exclude. It's quite useful in this exact situation.

I did try this, but for whatever reason it kept getting added back automatically. I forget the details of exactly why it was happening because it was close to a year ago, and in the compatibility guide it says this is supported, but I'm not sure if it was at the time or I was running into something different. This was a contract gig for me where I knew it would be ending within a month or so, which meant I didn't bother spending a ton of time trying to figure out a long-term solution.