QUOTE: “Starting April 21 (2015), we will be expanding our use of mobile-friendliness as a ranking signal. This change will affect mobile searches in all languages worldwide and will have a significant impact in our search results. Consequently, users will find it easier to get relevant, high-quality search results that are optimized for their devices”. GOOGLE

QUOTE: “Returning a code other than 404 or 410 for a non-existent page (or redirecting users to another page, such as the homepage, instead of returning a 404) can be problematic. Firstly, it tells search engines that there’s a real page at that URL. As a result, that URL may be crawled and its content indexed. Because of the time Googlebot spends on non-existent pages, your unique URLs may not be discovered as quickly or visited as frequently and your site’s crawl coverage may be impacted (also, you probably don’t want your site to rank well for the search query” GOOGLE

The reality in 2019 is that if Google classifies your duplicate content as THIN content, or MANIPULATIVE BOILER-PLATE or NEAR DUPLICATE ‘SPUN’ content, then you probably DO have a severe problem that violates Google’s website performance recommendations and this ‘violation’ will need ‘cleaned’ up – if – of course – you intend to rank high in Google.
In particular, the Google web spam team is currently waging a PR war on sites that rely on unnatural links and other ‘manipulative’ tactics (and handing out severe penalties if it detects them). And that’s on top of many algorithms already designed to look for other manipulative tactics (like keyword stuffing or boilerplate spun text across pages).
QUOTE: “Search engine optimization is often about making small modifications to parts of your website. When viewed individually, these changes might seem like incremental improvements, but when combined with other optimizations, they could have a noticeable impact on your site’s user experience and performance in organic search results.” Google Starter Guide, 2008
QUOTE: “Medium pages achieve their purpose and have neither high nor low expertise, authoritativeness, and trustworthiness. However, Medium pages lack the characteristics that would support a higher quality rating. Occasionally, you will find a page with a mix of high and low quality characteristics. In those cases, the best page quality rating may be Medium.” Google Quality Evaluator Guidelines, 2017
The SEO starter guide describes much of what your SEO will do for you. Although you don't need to know this guide well yourself if you're hiring a professional to do the work for you, it is useful to be familiar with these techniques, so that you can be aware if an SEO wants to use a technique that is not recommended or, worse, strongly discouraged.

If you are improving user experience by focusing primarily on the quality of the MC of your pages and avoiding – even removing – old-school SEO techniques – those certainly are positive steps to getting more traffic from Google in 2019 – and the type of content performance Google rewards is in the end largely at least about a satisfying user experience.

Google and Bing use a crawler (Googlebot and Bingbot) that spiders the web looking for new links to find. These bots might find a link to your homepage somewhere on the web and then crawl and index the pages of your site if all your pages are linked together. If your website has an XML sitemap, for instance, Google will use that to include that content in its index. An XML sitemap is INCLUSIVE, not EXCLUSIVE.  Google will crawl and index every single page on your site – even pages out with an XML sitemap.
When Googlebot crawls a page, it should see the page the same way an average user does15. For optimal rendering and indexing, always allow Googlebot access to the JavaScript, CSS, and image files used by your website. If your site's robots.txt file disallows crawling of these assets, it directly harms how well our algorithms render and index your content. This can result in suboptimal rankings.
SEO is the practice of optimizing websites to make them reach a high position in Google’s – or another search engine’s – search results. At Yoast, we believe that holistic SEO is the best way to rank your website because you focus on making every aspect of your site awesome. Don’t use any black-hat SEO tricks, because eventually, this will have negative consequences for your rankings. Instead, practice sustainable SEO, with your user in mind, and you will benefit in the long run.