Do I really think that's going to happen? No. If GitHub were to introduce ads or invasive marketing everyone would move to GitLab in a second. A decent amount of people are already moving to GitLab and GitHub hasn't even done anything! Things like marketing cookies for analytics on marketing pages are genuinely not an issue.
But people take it as a sign, because of all the shit that goes on the rest of the web, and how GitHub explicitly said they were not going to do this. The fact they made a "commitment" not to do this is particularly important because it shows that GitHub's promises don't mean anything.
But migrating your repo is easy. Personally I'm going to wait until things actually get bad before moving.
This already happens.
Do you think ads have gotten better or worse over time?
I think it's unequivocally worse now than it was then. There's a point where you're fed up with seeing the same ads for products you don't want or need.
Imagine if a store put a tiny sticker on anyone who stopped by to look at their products. Not this person is identifiable as "stopped to look at product X" whenever they visit other stores from the same owner.
Personally, it just creeps me out, even though it's basically the norm online.