The idea feels like sci-fi because you’re so used to it, imagining ads gone feels like asking to outlaw gravity. But humanity had been free of current forms of advertising for 99.9% of its existence. Word-of-mouth and community networks worked just fine. First-party websites and online communities would now improve on that.
The traditional argument pro-advertising—that it provides consumers with necessary information—hasn’t been valid for decades.
How exactly do you define advertising? An overly broad definition would forbid, for example, a dentist from putting a sign in front of their office saying they’re a dentist.
Paying somone else to advertise for you. You yourself holding up a sign peomoting yourself is fine, paying someone else to hold up a sign for you is illegal.