I'm going to mix quite a few things here.
Google’s position on the state of the Internet is murky to say the least.
arstechnica.com
The article headline is misleading. What Google is really saying is that ad revenue FOR WEB SITES OTHER THAN FROM SEARCH AND FACEBOOK/AMAZON is not rising. And furthermore that the court forcing Google to divest its own ad business that puts ads on other people's web site (the former DoubleClick, now known as Google AdX) would make that effect even worse. And that's an argument that seems plausible. I have no way to measure ad revenue (having been a Google employee for several years did not help in that at all). The amount and quality of information that's posted to the web and accessible there is definitely shrinking. And that's across news, journalism, arts, technical stuff, government, and even computers. The web was invented (by Berners-Lee at CERN) to have a convenient way to access technical and administrative documentation; it is in that function that it is more and more failing.
I consider classic forums like this one a part of the open internet. It is indexable by search engines, visible to everybody without login. So much better than discord, slack and facebook groups.
Facebook is searchable. Discord is at least searchable within a "channel" or whatever they call their top-most topics. Don't know about Slack at all. Reddit is highly searchable, and as other forms of authoritative content are vanishing, it is the either the successor to, or the last bastion of technical and discursive interchange.
You're right that forums are on the decline. Honestly, I don't understand why, compared to the success of Reddit.
And open internet in Europe is definitely coming to an end with the client side scanning requirement coming very soon.
Free communication has always been a thorn in the side of authoritarian governments. Book burnings by the Nazis, or the Holy Inquisition punishing people like Galileo or Jan Hus are historic examples. Current places like China, North Korea or Iran demonstrate how in centrally organized societies, information can easily be suppressed. And if you talk to "young" Chinese for example (under 40), you will find how much the filtering of news (such as about Tiananmen Square) has formed their consciousness, and not in a good way: for lack of an alternative view, they have absorbed the party's narrative.
The same thing is happening in nominally "free" countries that supposedly have "respect for civil rights". As an example, in Germany three leading politicians (Habeck, Struck-Zimmermann and a third one I forgot) each have filed over 2000 court cases against people who called them mild insults such as "Schwachkopf" (weak head) on social media, and in nearly all the cases they win the case and the person has to pay a 5-digit amount (in Euro) as a penalty. This very much changes political discourse there, and I can notice how people in public walk on eggshells, while in private they are more angry than ever at politicians. If you wonder where the votes for extremist parties come from, this is one of the reasons.
And even here in the US, which thinks of itself as a beacon of free speech, things are far from good. As an example, a friend of mine ended up spending half a million $ in court cases, after he tried protesting on Sand Hill Road in Menlo Park (the place where many of Silicon Valley's venture capitalist companies are located), against his ex-father-in-law, who is well known to be a child rapist. This didn't stop Menlo Park police from arresting him and roughing him up and charging him with some strange crimes. He ended up winning the cases and getting some of the cost of lawsuits and lawyers back, but it was still a dangerous and very expensive exercise. He doesn't mind.
Even I have been warned by my lawyer to not post obviously true facts (court documents, with signatures by judges!) on my personal website that would make political candidates or neighbors in my area look bad (I'm a bit involved in local politics). Getting that advice from my lawyer cost me $3000; I stopped posting such things, because my lawyer convincingly explained that while I would win in court, and I have every right to free speech, winning the case would cost me probably $30,000, and that wasn't worth it. So if you want to know about the misdeeds of Elise Moss, Carl Loeber, or Steve and Gina (Mikyong) Gray, you'll have to ask me offline.
The web is going into the toilet, together with free speech.
