Hank here from Shopify.
Thank you for posting your question, I was able to speak with our technical team about this to get you the correct information to answer this for you.
Currently, it is not possible to edit the subject that you would receive from when a customer emails you using the contact form option. I have checked to see if there is a way for this to be edited, but it is not currently possible via the theme or technical side of the platform.
What I have done for you though is I have logged this as direct feedback to our developers, as this is how the platform gets improved, with suggestions from store owners like yourself!
While I know this is not the response you were looking for, know that I have logged this now, with the hope that it does become a feature on the platform in the future.
Do let me know if you have any other questions.
All the best,
Thank you all for reaching out in response to this thread.
What I have done is gathered the feedback of every comment, but as it stands there is still no confirmation or timeframe that this may be implemented to the platform.
Thank you again for all of your feedback and for wanting to help build such a useful tool for our Merchants, it is very much appreciated.