Its complete nonsense that site maps are important... Its another myth that keeps perpetuating... Google and other search engines do a great job of indexing your website without them. As a factor which might contribute to rankings there is no actual evidence which shows that this is the case and hence its negligible at best... What matters is the type and quality of links (inbound, internal, external)...
Think about this.... A search engine crawls your website.... and discovers pages... Lets say you add a site map and for the vast majority there is no difference... i.e. Google still crawls the same pages... Do you think just because you have a site map your website will rank better? In the case where a sitemap provides additional pages which have not been indexed then you can be sure of a couple of things:
1) You have issues with your site navigation i.e. you have orphan pages.
2) Those pages most likely have zero inbound links and further zero link juice.
If deeply nested pages have backlinks then for sure they will get crawled and further quality backlinks are seen as votes for your site.... Again site maps are not needed....
The only situation when I would recommend a site map is for sites which have issues due to navigation i.e. derived links from a database such as a drop down list on a ecommerce site with it being obscured by code... But even then one should work to improve user navigation and in cases such as this I would recommend silos to promote such pages internally on a given site... Link juice attribution and how you sculpt the flow is hugely important!!! Site maps does not even come into the equation for that.....
Finally one important point - there are many reasons not to signup for a webmaster account such as when you operate a PBN due to footprints...
Bottom line sitemaps will not improve your rankings, traffic and popularity of your website... The absence of site maps for most will have zero impact given how search engines crawl links on your website... In fact its better for search engines to crawl links and index pages as they go along as then they can accurately determine the semantic relationship between pages and its relevancy.... and you certainly do not need site maps for that!!!