I think banner ads like the ones you are describing would be acceptable, I could definitely envision different Steem apps, crypto related products/games/etc, or even just users with large followings being interested in paying for some banner ads on Steemit. If I saw ads like those on Steemit I would think twice about it, they would be unobtrusive and entirely what one would expect when using a social media like Steemit.

I would still worry about how that would look to users. Unfortunately in this day and age optics are often more important than facts. A user will probably form an immediate opinion on banner ads when the see them, and that opinion will be hard to change even if that users takes the time to understand what information an advertiser may or may not have about you. I could see that affecting the way Steemit is perceived as a platform, especially where crypto is involved since people involved in crypto are often likely to be concerned with privacy than on a user on a traditional media site.

In summary, I would not have a problem with banner ads or sidebar ads, I could even see them enriching the platform, both literally through adding to the reward pool, and figuratively through providing users and Steemit apps a place to broadcast their products to the community. I would still be worried about how the presence of ads would effect the perception of Steemit as a platform and would probably still lean towards remaining ad free, but I also would not be disappointed to see ads on the platform because of the potential value they could add.