At the moment, we don’t have an activity posted to conversations when the subject is changed. While this isn’t something that we currently display, we do have an idea for this in our product idea portal here, and would love to see your vote added onto it.
Adding a vote and submissions of new ideas to the idea portal submit the feedback directly with our product team and also helps us share updates via email to share updates to its status and if we are able to roll it out in a future update!
While the application doesn't show you the activity of a changed subject line you can still see when the subject line changed. This has helped me be able to see if someone on the customers side made the adjustment on internally based on the subject lines history of each email received.