Apparently, it is true:
"In August 2015, at the D23 Expo, [John] Lasseter stated that the film would focus on the romance between Woody and Bo Peep. Its story will be built on the fact that Bo Peep was absent in Toy Story 3, with Woody and Buzz trying to find and bring her back."
My opinion: this doesn't sound like the best idea indeed.
Pixar has a way with things though, more then any other studio. It also might be pretty heart wrenching, like she went off to find a new owner, since the sister maybe didn't love her anymore or something. Maybe she broke and thought she wasn't wanted anymore? I am sure it will be good.