Both my boundaries and my therapist's boundaries have mostly come up in an experiential way as the need arises, rather than being talked about preemptively. Mine have mostly been relevant in terms of what topics are in-bounds vs. off-limits, and it tends to be the therapist who brings up those observations--e.g. "It seems like this isn't something you want to talk about, and we don't have to if you don't want to." Sometimes followed up by something like "But if you were comfortable talking about it I think it would be interesting and useful to explore." One time we explicitly discussed in advance whether I'd be okay with him bringing a topic up in the next session if I didn't bring it up first. Otherwise, it's not something where I think the map of boundaries needs to be discussed until we run up against one of them. But then again, I'm a very strictly boundaried person in general, so I don't need any encouragement to hold my boundaries where I need them to be.

In general my current therapist is extremely respectful of my boundaries, which is (slightly paradoxically) very helpful in considering that when he pushes them he genuinely has a good reason for it.