Arduplane 4.2 and above no BRD_ALT_CONFIG parmeter

Hi using omnibus v6 and v6.1 nano after apgrading from 4.1.7 to 4.2.0 and 4.2.1 BRD_ALT_CONFIG parameter not available need this for extra serial. Downgrading to 4.1.7 this parameter available again. After setting it to 1 and upgrading to 4.2.1 serial 3 works on v6.1

Looks like a bug?