My understanding is that they're switching to a paid model. As in, you'll have to pay to continue using it, but if you do start paying, it'll work indefinitely. (Or at least until the makers of Infinity make the determination that even having users pay won't be enough to keep Infinity financially sustainable.)

Given that that's Infinity's plan, the theory is that probably the makers of Infinity have gone to Reddit and negotiated an extension of the non-paid API plan long enough for Infinity to implement a way for users to pay for it. I don't think there's any official word exactly how long that extension will be, but the expectation is that it will run out at some point and when it does, you'll have to pay to keep using Infinity.

One thing I'm not sure about, though, is how exactly that'll work given that Infinity is open source. Surely there's a "shared secret" or something involved. And for that kind of authentication method to work, the secret has to stay... well... secret. So they wouldn't be able to just commit that secret to the Github repo. Maybe it'll be some kind of OAuth2 scheme or something where Infinity-owned servers and Reddit servers will communicate behind the scenes to get you logged in.