I read through the search you linked, and it was an interesting journey.

I'd say that I'm not sure the search you linked supports the idea that people have been asking about a decline in quality in Google for 13 years continuously. Most of the threads about that topic seem to start about 4 years ago and pick up in frequency until the now.

Though this could just be a result of the search engine's limited ability to index old threads or some other sampling bias (like there being more users on the site recently).

All of that said here's roughly what I saw in the threads you linked:

2009: People notice that advanced settings in search seem to not work as well as before.

2011: It seems like Google now will start to ignore some of the words in your query if they're uncommon.

2011: Google starts giving preferential treatment to its own products like Google Reviews, Shopping, and Youtube.

2012: Users note Google Plus's "+1"s seem to have an outsized influence on search results.

2013: Google Hummingbird launches - it now appears that your query always gets passed through an interpretation layer. Making precise queries becomes significantly more difficult. Note this was probably (?) to make voice search return better results since those queries were more conversational.

2015: The importance of site performance in general and mobile specifically seems to rankings seemingly increased significantly. Likely influenced by Android One project.

2016: Google increases the importance of following their "webmaster guidelines" and W3C validation.

2016: Amit Singhal who oversaw Google Search for 15 years leaves the company to join Uber.

2017: Google starts to try and identify and deprioritize "offensive", "upsetting", and "inappropriate" content, as crowdsourced from users and paid evaluators.

2018: Google launches an update aimed at websites with low "expertise, authority, or trustworthiness." They state that they punish sites which are "not in line with the general scientific consensus" or which have negative sentiment associated with them on review sites.

- It's about this point that threads about Google's decreasing search quality show a strong uptick.

2019: Google starts using BERT - a machine learning model for interpreting search queries.

2020: Google releases updates aimed at further reducing the rankings of "misinformation" or "biased" content.

- At this point there appears to be strong consensus, even outside of technical circles that Google's search results are deteriorating. To the point where there are memes and popular Youtube videos about it.

2021: Google releases MUM - an AI/ML model that aims to reduce the number of queries a user needs to make by including results for related queries that it thinks you'll make next.

2022 (May): An update to Google's search algorithm deprioritizes reference websites like dictionaries, lyric websites, and wikis while promoting video content.

2022 (June): After years denying that a pages 'freshness' affects its ranking, Google increases its importance in the algorithm dramatically - reportedly affecting the ranking of 35% of search results.

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I'm not a search engine guy and don't have any particular knowledge about Google's search engine. But reading through the history of HN discussion about it does paint an interesting picture.

I'll leave it to others to speculate about any correlations between these changes and the end product.