We have discussed this a bit more in detail in the linked section:
ABI of plugins may change at every release, i.e., also within a stable ros2_control release. Plugins built by the buildfarm will still be loaded by pluginlib’s class loader, but code linking against the exported libraries will break.
For non-plugin code, ABI breaks within a stable ros2_control release are less likely but still unavoidable to fix code, which is critical regarding safety aspects of robot control.