As someone with development experience, I’m not even sure it would immediately occur to me to check the page source. It also is not really a realistic option on mobile (which most of my users are) and makes an assumption that the users knows where to look in the source (since they can’t just ctrl+f the software name if that’s what they are looking for)
Additionally, Discourse seems to primarily be used in the tech space, so many of the people with the ability to check the page source can already recognize the Discourse UI. For the vast majority of non-technical users, there’s no realistic way for them to figure out that the platform they are on uses an open source software called Discourse
+1 to on by default +1 to keeping it optional