Gregory's Server@moffintosh popular accounts with lot followers can still add advertising posts into their timeline, but they would really only on voluntary followers to see them. The instance operator may ask such accounts to pay some fee.
If fediverse users don't like the content vs. advertising ratio of such account, they would simply unfollow.
Popular accounts may even just simply link to their sponsors from their profile. In fact it is not that much different from how advertising worked on FM radio, or so. It just cannot easily target users based on knowing their personal preferences, like centralised social media (but as fediverse ecosystem evolves, this may not be true any longer ... except for accounts who really hide everything and just consume)