The notion of "boundaries" is a relative newcomer to the therapist-client relationship, in my 50 year experience as a client/patient.

I think it has to do with the notion of "ego boundaries", which some of us clients don't have very well. Either too amorphous or too rigid or something. A poor or false sense of self. So the idea, I think, is that the therapist will model what having "good boundaries" is like. Also the therapist needs them to take care of himself/herself.

I see absolutely no reason why a client shouldn't/couldn't establish their own "boundaries", either at the beginning of therapy or part way through if they begin to get a sense of themself and their "authentic" ego. I expect, but don't know because I don't have any, that "healthy" relationships involve some knowledge as well as give and take about the boundaries around the people in the relationship. So it could definitely be to the client's advantage to get practice with that in therapy.

If the therapist doesn't agree, then that's a pretty good sign. . .or red flag.