I've been with my T for a long time. In the beginning, she barely disclosed anything. Her boundaries have loosened over the years, and she let me know some things about her; also, we have some mutual friends. I also do my fair share of googling her, and she knows this.
Since her cancer diagnosis a few months ago, she has been open to anything I've asked her. My therapy is about me, but part of that is her experience of cancer, her procedures, treatment, side effects, prognosis, and that sometimes we have to change my session schedule (every 2 weeks), because she is having a procedure or treatment done, after which she needs to recover and rest for a week or so. These interruptions in my therapy schedule do affect me, and we talk about that. We've talked about my fears and her fears, and she knows that I'd want to know if something drastic happened. Some of these discussions have been extremely hard, at least for me; my T seems pretty relaxed in session, although she confirmed for me that she had "some moments" at home: being stunned at first, etc.
I don't know if she is as open with her other long-term clients, but I can see that she is being this open with me because of my issues with neglect/abandonment/attachment, etc., issues and fears. She said I can ask her anything, even things of a private/personal nature, and I've asked a few questions about those things, albeit I was barely breathing, whispering, almost crying, and feeling embarrassed at the same time.
This is a very complicated experience for me. Sorry for rambling.