The internet was never ad free. The first ad was posted online in the 1970s (for DEC)! It pissed people off but not everyone: supposedly it generated $18M in sales. There was very little advertising back then only because the internet was restricted to a handful of large companies and universities.
The web itself was launched in 1991 and the early web was inaccessible to basically everyone as it required an extremely expensive NeXTStep machine. Windows didn't even ship a TCP stack in this era, iirc. So took a few years for the web to reach the point where it was usable at home. By 1995 the web was starting to become barely usable thanks to Win95 and Netscape, and DoubleClick launched immediately in the same year.
My memory of the early web is that basically every website had DoubleClick ads on them, it was notorious for that. "Punch the Monkey" was an early campaign. Almost every topic oriented website carried ads, partly because bandwidth and servers were very expensive so that helped defray the costs. GeoCities took off because it handled the complexities of running ads for you, so you could publish for free.
Companies would announce job openings. These announcements were restricted to a few newsgroups such as ba.jobs and could not announce independent contractor openings: while it was okay to announce an opening for an employee (W2) or for an individual to announce his availability for employment, it was against the rules for a company to announce an opening for an independent contractor (1099) or for an independent contractor to announce his availability for work -- that was considered too much like commercial activity and was kept off Usenet (which was definitely the "front page of the internet" at the time). Usenet had almost no rules in the early 1990s, but this was one clear rule that was consistently enforced: no commercial activity.
There were also announcements of academic conferences of various sorts, and these were much more annoying than the job announcements because they did not remain contained in a few newsgroups (all of which IIRC ended in ".jobs": it was considered OK for example to post an announcement for an upcoming academic conference to for example a programming newsgroup if the conference had anything to do with programming.
But as far as anything ad-like, job announcements and conference announcements are all I can remember.
I heard that the proprietary online services like Compuserve and Prodigy had successfully argued in Washington that it was unfair for private enterprises to have to compete with a government-subsidized service (namely, the internet) and extracted a commitment from Washington that any commercial activity would be kept off the internet.
Again, there were almost no rules on the internet of 1991 and earlier. Tim Berners Lee for example did not need to get anyone's permission to deploy the web: he could just write a web client and stand up the first web server and announce their availability (using Usenet and mailing lists). But one very well-enforced rule was no commercial activity, e.g., no selling, no trading, no advertising.