This guide is specifically for migrating existing devices, not about new devices. When creating new ABP devices on V3, you can request a DevAddr from the Network Server, and Packet Broker routing (at least of uplinks) will work fine. If the firmware of your ABP device is configured with an RX1 Delay of 1 second (which it is, unless you changed it), downlink through Packet Broker may not work (it may not arrive in time for that 1 second window).
People are free to migrate their gateways, or to keep them on v2. We do ask people to coordinate the migration of gateways with their local community.
And I can’t repeat this often enough: We’re asking people to not use ABP devices unless they really have a good reason to use ABP.