HBO Max "thanks" me for being a loyal subscriber, by removing features from my plan without reducing the cost.

Legality isn’t even a thought for companies anymore, and the punishment is only a small percentage of what they make for screwing their users. All they care about is profit and saving in server bandwidth by making quality worse.

Probably, but if not...

Publicly Traded Corporations that have purchased their own regulators through legalized political bribery: We will make it legal.

Yes. It's like a contract. (Just double checked the date and it's >30 day notice)
In this case, Max is letting them know that on Max's end their changing the terms of the contract. The new terms are Y.
Customer can opt to not continue the contract before the changes go into effect.

Here in the US, on a cable internet provider we essentially do the same thing every 2 years when we come off promotion pricing. I call, tell them what I'm willing to pay and they decide if they're willing to provide service at that rate.
They decide if they are willing to accept my terms or offer different. If they offer different, I call up the other cable provider and take their 6 month promo and tell the first one to come get their shit.