This isn’t true and this level of cynicism is really unhelpful.

They originally used Gmail in that way. Many years later they stopped. This was a major victory for privacy. The fact they seem to be doing it now again is bad. Saying they’ve “always done this” is the kind of empty conspiratorial cynicism that dampens action and makes people feel it is futile to notice these changes or speak up about them.

The fact that they did it in the beginning and are doing it now makes the whole “well, there was a period of time when they didn’t do this” seem like a minor footnote to the notion that “they’ve always done this”. Even though the statement isn’t strictly true, it sure seems like the cynicism is warranted now and, with the benefit of hindsight, was back then too.

What's the evidence they stopped?

I don’t think it’s conspiratorial to question how strong the firewall was between your Gmail content and Google’s algorithms. Whether or not they actually leveraged that understanding for targeted ads is somewhat beside the point.