The boss power tab wasn’t available to me despite reaching level 60. (I thought wrongly it unlocked when you finally got to T1 but read that it should be available since the beginning of the game.) I saw a hack where you could start a new seasonal character and do the first part of the seasonal quest and it unlocked it for the character it was greyed out for. I did that and it unlocked it for me.
However, since then no boss powers from Incursions or World Boss’s are dropping for me.