Almost everything gets "removed" from the radar every six months. The Thoughtworks radar is published twice a year, and each volume of it only contains what Thoughtworks found new and noteworthy in the six months leading up to the publication of a volume. There's more in the FAQ: https://www.thoughtworks.com/radar/faq-and-more
In my opinion the reasoning why "SPAs [are] way easier to reason about in the abstract" is neglecting to acknowledge that most applications evolve over time.
There is no well-defined API on the server side. The application logic is smeared across code the runs in the browser and code that runs on a server. That "API" is usually merely an HTTP endpoint that connects both sides. Adding new functionality to the application requires touching both sides, and changing the HTTP endpoint. I would use different approaches to build those HTTP endpoints and to build a "proper" API that's used by different consumers.
