I've never heard this before. Is there any evidence (not speculation) that this is true?

It was well known for us early gmail users that they would scan mails (on-the-fly iirc) to provide relevant ads.

They were fairly up front with it (remember, back in the days Google was a nice trustworthy company or at least that was my impression.)

Is there any proof that Google does not? Scanning user's private data to target them with ads is the primary reason Google offers services. Absent other evidence, the default assumption should be that Google scans as much data as they can.

Proving absence of something is an unreasonable standard. Your answer does sound like "no, there is no such evidence" to my ears, though.

I guess I wasn't clear. I had something in mind like a public statement from Google saying that they do not scan a particular set of data. I would accept that as evidence. Absent a statement from them either way, I assume they read whatever is uploaded to their servers.