Barred Plymouth Rocks do "breed true", for breeding true is essential to be declared a breed. The concept of "true" needs to be examined, however. True to what? What should it be "true" to?
To the Standard written for the breed. In the US, from whence this particular breed originated, that Standard has not been substantially changed in over a century. It took a bit of time to settle on the angle of the tails and a bit of time to settle into agreement as to the width percentage of black to white, but those are minor, really. Also, very early on, the determination was made not to continue with two varieties, rose combed and single combed, with the single comb winning out.
The strain you have is essential understand and get to know. A particular strain may have flaws and faults that are not the best representation of the breed's written Standard. It simply means the breeder has work cut out for them in subsequent generations.
Breeds are NOT species, and thus, they are man-made composites of various birds used originally to create them, Cochins, Java, Dominique, etc. This composite will not stay together all be itself. To maintain a breed means a breeder must work hard, at every generation, to maintain the birds to the written Standard, or the genetics will drift and the birds will soon wander off into whatever their genetics wish, and soon, they are no longer good examples of the breed. Sometimes, skilled breeders will introduce some genetics or "blood" from another breed, not to carelessly pollute the strain, but in hopes of returning it to a better example of a Plymouth Rock. Not all these tweaks are successful and subsequent generations end up farther from the Standard rather than closer to the Standard. Such experimental birds should be noted and in most cases should be culled for eating and not sold to unsuspecting buyers who think they are buying true bred Rocks and breed them indiscriminately.
A bird is said to be "pure" not because of it's blood line or pedigree, such as in horses or dogs, but because it LOOKS like the written description of the breed's Standard. That is what makes a "pure" Barred Plymouth Rock, it's shape, it's form, it's coloration, etc. When these things present correctly, the bird is "pure".
BTW, every nation decides for itself what Standard their poultry breeders will subscribe to in this fancy. The Standard in OZ is slightly different from that of the APA. Hope this helps.