There are many factors at play here, some of which including:

• AI content is taking over the Web: with the popularization of LLM tools, there's an increasing number of AI-Generated content across the Web. Even press websites are using them for generating news and opinion articles.
• Old sites/articles are vanishing from existence: for instance, old blogs and personal web pages, which contained a lot of useful information, are being deleted due to factors such as domain expiration, hosting expiration, insufficient web traffic for the host to keep it online, etc. To make things worse, few of these sites were archived with tools such as Internet Archive and Archive Today, meaning that, when they disappear, they really disappear.
• Dominance of Reddit-owned contents and the Reddit issues: Reddit doesn't need introductions, most of the questions and content used to come from Reddit posts and comments. Things such as people (understandably) deleting their Reddit accounts make content to disappear as well.
• SEO bs and marketing spam: Google kept changing "page ranking" algorithms, sorting results according to their own will. "Search Engine Optimization" is a just a facade that led many old sites to practically vanish from search result pages. Advertisement also did harm many sites as well, even the bigger ones.
• Societal, economical and human changes: there were lots of changes upon society and humans by the last 5 years. These worldly factors also influence the digital landscape.

That said, it depends on what you're searching for. If you're searching for knowledge that used to be at old websites, you can use Marginalia to search this specific type of websites (considering that they're still online).