I think it's almost definitely a myth — one that will probably never die as I think it's in a book or two — that Lainie Kazan was fired as Streisand's understudy. She was in the job for at least a year. She did have a good rep who was getting her lots of publicity as the understudy who had never gone on. Then she got more publicity when she did go on. I think it's a myth that anyone reviewed her. I find no evidence of anyone reviewing her. (The same myth is out there about Jane Romano, Merman's standby in Gypsy.)
By the time she finally did get to go on for Streisand, Kazan had gotten enough attention that she was soon playing nightclubs. I think it's quite possible that they didn't renew her contract, but I think it's also possible that she didn't want to renew her contract. In any case, I think it's unlikely she was actually fired.