I don’t know how this rumor started (I’ve seen it on several forums in the last six months) but fortunately that’s not how it works.
All generations of zwave are backwards compatible per the spec, but it was always intended that you could mix and match generations without crashing the whole network back to the previous generation.
Instead, each individual network segment between any two devices can take advantage of the capabilities of those two specific devices. If they are both zwave plus, an individual message sent between them has full zwave plus functionality.
If one of them is an older generation, you might lose some functions—but only for that specific segment . The zwave plus device could still use plus features on other segments if the other endpoint on that segment was also plus. And nothing changes the range of the individual radios. If the plus device can transmit for 75’, it can still transmit for 75’ even if it’s talking to a device that can itself only transmit for 40’.
So feel free to mix and match generations, but to take full advantage of new network management features like improved network wide inclusion try to position Devices so that each plus device has at least one path back to the hub through other plus devices. Otherwise don’t worry about it.
Oh, and run a zwave repair anytime you add a new mains powered device of any generation.