When I was younger ì sat down with a therapist ,who of course was a complete mismatch , I was there to work on severe abuse issues of childhood, just really someplace safe to "emotionally throw up" , they proceeded to tell me they don't talk about the past ,they don't care about past issues, they only work on things that effect your present life .

Call me an idiot but our past experiences typically have a huge influence on our present life as well as our personality and our reactions to life events .

So I said thank you No, your not what I am looking for ,nor someone I could develope any kind of therapeutic relationship should I need it .

Ultimately this guy persisted at being a jerk ,and showing up in my life wear he didn't belong and wasn't welcome ,I ended up filing a PFA against him to get him leave to me alone .

The relationship never would have worked and me telling him F no should have been enough ,it was a meet & greet session , that was an on ramp for the "crazy train" , so i firmly believe in telling them both what your looking for, what you need, and what kind of past therapy was most helpfull or effective ,if that happens to mean disparaging dialoge about them or past T ,so be it .

I have a boatload of physical health problems so I have like 18+ doctors, when one crosses a line ,I warned them not to cross on day one,I fire them .initially I felt it was unfair to talk about the idiots that led me to there office , but now I spell out exactly why I fired the last guy , yes "boundary violations" is a frequent issue .
It doesn't guarantee that the new guy will be better or smarter ,however the trail of "bodies" of past doctors and why they are past doctors can help them know what not to do ,I feel the same way about T or anybody else you consult .

If you hired a plumber to fix your toilet and they broke your sink ,would you pay them and keep silent ,I don't think so ? So telling the truth as you know it is perfectly appropriate .