It has been renamed to getConfigBool in https://github.com/pmmp/PocketMine-MP/commit/b6264d188ecf1d5e8f0915d29601d7e92a9d51d8
getConfigBoolean() was deprecated several months ago and has been removed in the recent development builds. A release will be created soon™️ which will document the API changes. In the meantime, please avoid using dev builds and use the tagged releases instead.