Isak has not yet made any official statement about leaving Newcastle. We have not even seen any such gossip yet. Liverpool made an offer to buy Isak for 110 million. But Newcastle management rejected it. And Newcastle management does not want to let Isak go now. But Isak has not said anything about his desire so far. However if Isak wants to leave Newcastle, Liverpool may be the best option for him. Liverpool's squad is very strong this season. With Isak's help, liverpool may be able to dominate both the Premier league and the Champions league.
Isak won't make official statement about leaving Newcastle until his current club will able to find his replacement. However, he's in stance to leave from the club. I saw the latest rumor says his club targets Jackson as his replacement, but still waiting for more concrete news to come. If his club successfully sign Jackson from Chelsea, he can leave.