I know what you mean, snowberry, but online discussions work because we are anonymous. Our opinions speak for themselves and are not coloured by prejudices or perceptions about what we look like, how we speak, where we live and so on. If we know a lot about one another that dynamic shifts, and people start to become more guarded about what they say. Also, if someone said she is trans, everything she said after that would be viewed through that prism, which would inhibit debate - particularly for her, but also for others talking to her, and her experience of GN would change. Some people would be fine with that but others not, and it has to be their choice. And, of course, there is the possibility that the hypothetical trans person would be ’outed’ offline, whether she liked it or not.
My life is entirely unremarkable, but I am careful not to give too much detail in case someone I know recognises me. Not because of a big secret, but because I might be less inclined to be honest about my posts if I thought they might identify others (eg mentioning colleagues or family members, or giving examples of things from my life). Again, no big secrets, but it would be awkward and unfair.