In order to most-to-least charitable, any of:

1. Github could choose to grandfather in those plans and make no changes until those plans expire.

2. Github could offer, or the user could request, a pro-rated refund along with cancellation of the account.

3. Tough luck, those users agreed that Github could unilaterally change the ToS at any time.

I doubt you can force them to provide the service with the original terms, but you might be able to ask for a (partial) refund. If not today, after a week of verbal abuse they will receive for this online.

In places with reasonable consumer protections (Australia, Germany) it almost certainly is illegal unless they give a full (whole year) refund. I think the short time limit of applying for a refund won't be looked at favorably either. Regardless of their ToS which I'm sure covers this.

But companies do lots of illegal things, and in general nobody takes them to court over it.

My thought exactly! First the usage limits + model limitations and now fundamental change to the billing. Hope some consumer watchdogs are looking into this!

100% it says in their terms that they can change the service during the agreement.

I just checked and you can cancel with a refund.

For the yearly plan they only change the model multiplier. And it's in the subscription contract they can change that multiplier at any time.