They weren't the centerpiece of the episode, like Jack Black as the new comic guy

they didn't use their real names of course but you could tell it was them

And those were the exceptions and not an every episode occurrence. Some of those also had good points to make like Lisa going vegan. Very much not how the modern cameos go.

Somewhere around the point of Lady Gaga driving into Springfield on a magical train it became celebrity ads.

Absolutely. I was watching the series sequentially for the first time in ages and that episode stood out as a definite turning point. The celebrity appearances just got more frequent and more insufferable from then on.