It wouldn't be in Google's best interest to do so. First, it could be used by spammers to lookup their own reputation for a given IP or domain before deciding whether to move on. Second, it would enable competitors to use some of Google's hard work in their products.

Wait! Maybe not so fast dismissing this.. G already does this, with web search. It ranks reputations and its publicly available.

Trying to game the system in web search is a huge thing, cause there's a big reward. But G fights it pretty well.

Not such a bad idea G publicly ranks mail senders.

Too easy for spammers to utilise to game the system. Just make a new domain (reputation 0) then keep on sending different emails to see how the reputation changes. The linked reply kind of covers this (e.g. security through obscurity).

I don't believe Google wants to turn GMail into a data broker service, especially one that powerful. Why? Besides the obvious (spammers use it to learn how to beat the system, and other companies use it to compete with GMail) you'll be creating an insane amount of workload in having to deal with all the complaints. The current system of "spam is what our users say it is" leaves very little for debate. As someone who's run large email systems this appeals to my laziness.

I wonder if Google uses "shadowbanning." If a spam account emails another spam account, is the email filtered?

I know that's probably not why it's private, but it got me thinking.

GMail's size and centralization is its competitive advantage. It's not in Google's interests for you to benefit from its data mining efforts without also contributing to them.