Interesting, but it’s hard to see the big utility. Most spam signals are based on behavior and reputation rather that content. Your email can look clean at first, but if a handful of gmail accounts mark it as spam then your whole network will be on the bozo list within seconds. Gmail in particular is capable of reclassifying messages, if it gets delivered but then your reputation falls, it will get moved to spam if the user hasn’t seen it yet, so initial deliverability is not final.

Maybe the other big operators are more content-based?