The Commander Rules Committee has released a statement in response to today’s Banned and Restricted announcement, which changed the way the companion mechanic works.
Once per game, any time you could cast a sorcery (during your main phase when the stack is empty), you can pay 3 generic mana to put your companion from your sideboard into your hand. This is a special action, not an activated ability. It happens immediately and can’t be responded to. It can’t be countered or stopped by cards like Phyrexian Revoker.
Beyond the changes outlined above, the Commander RC does not feel that any “additional changes [are] necessary” before the next scheduled update, currently planned for June 22. Companions will “operate in Commander similarly to other formats”, but, naturally, will not be pulled from a sideboard.
Read the original article from the Commander Rules Committee.