Charlie Brown:
In the case of remission I was not going to say outright before they met, though I would answer truthfully if asked
If ANY shadchan set up my child with someone who had been seriously ill (even if B”H in remission) and DID NOT tell me prior to the shidduch being made and get my informed haskama, not only would I not trust that shadchan ever again, I would tell my friends not to utilize that person’s services. A lie of omission of that magnitude, is not the same thing as lying about a girl or boy’s age (bad enough to do that). It is a rachmanus that a young person is chalilah ill, and they deserve the happiness of getting married, just as anybody does. But to fail to inform someone that the boy or girl was once very seriously ill with a potentially life-threatening condition, well, that is just unethical in my book. What if the girl and boy were to really like each other, and THEN find out? Now you have the potential for two heartbroken kids, should the well-person not want to take the very real risk of being widowed at an early age (and although people could get hit by a bus crossing the street, yada yada yada, this fear of the cancer returning is far more realistic a concern, given the circumstances).
If this were not a problem, there would be no need for Dor Yesharim, either. Why should anyone be told in advance if there is a genetic disease in the family? Because, to NOT tell, could result in a child being brought forth into the world with a doomed future chas v’sholom.
I understand that not all things have to be told – it is irrelevant if the uncle on the mother’s side lost his job, or there are non-religious members of the extended family. These things can be discovered in the course of dating and really have minimal impact on the couple, for the most part. But if that same uncle is bi-polar, and there is a possibility that the prospective shidduch might also be, the other party has a right to know before going forward with this for even one date, IMO.