I emailed tesla about this a while back. I got this in response.
Thank you for contacting Tesla, I apologize for the delayed response. There is not a backslash button for the Model 3 at this time. However, I went ahead and submitted a feature request for this character to be added to the keyboard options. I cannot promise that this feature will be available, but our team reviews every customer request seriously.
Please let us know if you have any other questions or if we can assist in any way.
Thank you for helping us accelerate the world's transition to sustainable energy.
This is still an issue. I was trying to log in to a website the other day and discovered that there are no \ or | (pipe) characters. Seems the special and punctuation characters set is somewhat incomplete. This is in a Model 3 running 2020.4.1. I considered finding the character on a web page somewhere and then copy-pasting it in but that's not an option on the Model 3 UI, either.
Now I'm wondering if the Model 3 would recognize an external USB (or bluetooth) keyboard...