I can't think of anything where my T didn't disclose something I was asking about, but I also can't think of questions that might be 'boundary crossing'. I'd not be comfortable myself knowing where my T was going or what he was doing during his time off.
My T has mentioned far more about his personal life by himself (like how many kids he has and what gender, certain issues they have, what kind of music he likes...) than what I'd be comfortable asking about.

He does sometimes mention things that he doesn't disclose though, like 'an event that a group of friends and him went to' or that he has experienced 'similar' circumstances at certain points in his life. In those instances, it always felt like it was to not make it about him or his experiences.

I'd imagine there's certain boundaries to what I could ask, but I think if I asked something like what kind of sports he likes, he'd probably answer if we were mostly chatting and he'd not somehow think that I'm trying to avoid something else.

Disclosure of personal information is difficult with therapists since it can easily get to a point where it's about them instead of the client or where the information in some way hurts the client. I for example react a lot to personal things my T shares, most of the time not in a good way. This can be a very hard line to walk. Information like this can also change how you view your therapist unnecessarily. Imagine somebody really hates tomatoes and suddenly learns their therapist loves tomatoes. That's maybe a bad example, but this could really change the opinion somebody has of their therapist, which is neither helpful nor something that is supposed to happen, it doesn't matter for therapy at all. So sometimes not sharing stuff is a better decision.