This has nothing to do with a monopoly, so antitrust law is totally not applicable here. You might be thinking "class action lawsuit", which someone might attempt. However, if Apple can show that this is actually a mechanism to provide maximum screen time for the life of the battery (as opposed to malicious planned obsolescence), the suit would likely fail.

I mean in Europe. Class action does not apply but EU has a strong record of defending consumer rights.

The clandestine nature of how apple has provided this feature would not work in their favour.