I had a feeling this would be your outcome. The change in SEO status is a big surprise though. Thanks for reporting back.
I run a large 13 year old forum for electric bicycles and i suspect my pending move to xenforo will give me a similar improvement, although our SEO is shockingly amazing considering we're running phpbb with zero extensions installed to help SEO out.
I really wanted to use vanilla forums and hopefully contribute some code to the platform. But a non-responsive dev team is a sign of a dead project to me.
I was once a financial contributor to flarum until i started poking into their source code and realized it's the kind of multiple layer maze many programmers tend to design with laravel. I pointed out that this design makes flarum unsustainable in the long term because this makes the code exponentially harder to understand as it grows. Nobody agreed with me but i saw progress on development start to perpetually slow. The lead programmer left the team shortly after with no explanation as to why.
Discourse is a decent platform that runs on a language ( ruby ) with no future, a small userbase, and very poor performance. Being started by the guy who invented stack overflow gives it great street cred. We investigated it as a candidate for our large platform and nixed it a long time ago. Perhaps Ruby 3.0 will provide halfway decent performance but that's a while away. Development and support of Discourse is pretty good, however their choice of programming language makes their forum more expensive to run than many others. I wouldn't recommend it to anyone unless Ruby 3.0 performance really leapfrogs Ruby 2.x performance.
While I don't necessarily disagree with everything that has been said here, I would like to stress, that the assumption that search engine performance is reliant on the forum software used (which has been repeated multiple times in this thread), is mostly wrong.
If you believe, that a different HTML structure or more meta tags or some kind of techy "magic sauce" can somehow improve your google ranking, you have bought into the SEO snake oil.
Sitemaps, meta tags, etc. are all a thing of the past as far as SEO goes. Major search engines do not just download your HTML source code. They execute and scan the page in a manner a human would, because that is their job.
That is most likely because your content is great. Choose a forum software for its usability and design not for "SEO" features.
Search for "the greatest page in the universe". What you'll get as the number one result is maddox.xmission.com. He named his PAGE the greatest page in the universe, so when you search for it you find him.
It is ironic that you are taking this site as an example, which is basically an iconic HTML 1.0 era site, known for its satirical articles, which has built credibility with search engines for two decades. Why should your recently created page rank in the same way?
SEO is all about content, that is the "uncomfortable" truth.
You need good and relevant content which attracts users to earn your ranking. This requires time and knowledge about the target demographic of your community/website. This can include having a good forum structure and teaching users how to choose comprehensive discussion titles, for example. It is not something that can be automated.
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I have decided to move away from vanilla but it has nothing to do with SEO and more to do with security and lack of support. The forum that I did have has not had a single security update in more than a year (from this forum) and I have noticed that lots of requests for support are going unanswered and spammers are even taking over.
I disagree about site structure having an impact...
If your site structure is very bad.. it will dock you in SEO big time.
I know this because i have one client that made their entire front page without any consideration to SEO and did not use a CMS like wordpress. This client's website had been around for 20+ years. Our SEO contractor put about 15 hours into restructuring the frame of the website, and barely touched our content.. the company's business has been up 20% ever since. And google analytics reflected the change fairly quickly.
Looking through the HTML of this site, i see how a search crawler would be very confused by Vanilla... the meta tags don't even describe what the thread is about.. what is content and what is not is extremely poorly semantically described. Seems there was no intention to create good SEO. It looks worse than that company's HTML code.
Quick overlook of vanilla's HTML structure:
There are 3 different title tags. There should only be 1.
Meta descriptors are not consistent and not all of them are relevant to the page content.
There's scads of div classes used to describe most of the content instead of semantic tags.
A page load of this thread is slowish at 2 seconds.
There are lots of individual css/js files and a decent amount of inline scripts in places they are not necessary.. which is a no-no.
Quick overlook of Xenforo's HTML structure:
Xenforo's page load speed is ~50% faster and the use of lazy loading is a big factor in this.
The meta tags, especially descriptors for facebook and twitter, are dramatically better.
There's only one <title> tag.
css/js includes are compacted into smaller files ( google likes this )
semantic tags like <article>, <aside>, and <section> are used.
I'm not an SEO expert at all. But based on what i've learned from my SEO contractor.. vanilla is not designed well in this regard. Xenforo seems to have made a decent attempt of making the HTML code machine readable.
@neptronix are you a Xenforo salesman? Xenforo costs an arm and a leg, you have to pay for plugins and you have to pay again every 6 months or so.
...but you are right Xenforo forums do seem to rank well.
[...] making the HTML code machine readable.
There are many other upsides to clean HTML code of course. Googlebot however, actually renders your page using the latest Chromium engine. It sees what a human visitor would see.
If there were invisible elements to SEO you could game the system. If a Xenforo forum with the exact same content would somehow rank better than a phpBB forum, that would be a mistake on Googles part. With phpBB still being the most used forum software, it should be able to parse its content just as well (if not better).