[mjn]

Yes, treating dialup sources as likely spam sources has been done since long before Google became a significant player in email. In earlier years the main approach was to try to convince ISPs to filter outgoing port 25 by default on their dialup IP ranges. Later, people started compiling lists of dialup IP ranges (later expanded to DSL/cable/etc.) to block them at the recipient side, since there were too many ISPs who weren't filtering. Recipients disliked email from dialup IPs because ISPs seemed unwilling or unable to police their customers and respond to individual abuse reports, and so little legitimate email originated there anyway that it was easier to just cut them off.

I don't think the bigger end-user providers have been very involved in developing those kinds of policies. The NANAE crowd was/is mostly administrators of smaller and university servers, not Yahoo/AOL/Hotmail/Gmail administrators.