Is it safe to let my cat outside?

For context- im trying to get my kitty used to the outdoors so its fun. Shes about 11 months, (12 on the 16th) and she was a street cat when she was a small kitten we dont know much about her childhood though. I put a harness and leash on her indoors and she was not used to it and just sat down wherever i put her. Im not planning to let her outside anytime soon, just getting used to the leash so far and giving her treats when we put it on, but we have a fenced in backyard but i am really scared that shell get spooked and jump the fence or something. Im here to ask if that's likely or if she might somehow get the harness off