I wonder how this will affect apps like Apollo, which can still run fine if you use your own API key, but wouldn't be able to publish compliantly on the App Store while giving users the capability to use their own key (since it violates Reddit TOS).

More generally, will this lead to a resurgence of "unofficial" third-party apps for popular platforms like Instagram and Snapchat? These companies rely on the App Store to be the gatekeeper that stops such apps from proliferating. But if Apple can no longer control that, then I expect we'll see more usage of them. Perhaps Meta already understands this and sees it on the horizon, which is why Threads is built for the Fediverse, where third-party clients are the expected and default mode of operation.