Google has done a change in 7 June 2019 and it is that you cannot do more than two-page listings on the same website. This change is Site Diversity change. Earlier, you listed all pages of your website or two pages on the same website but now it will be removed by Google.
Site Diversity is just a change, no updates.
This page will show how the web pages will be displayed on the search result but it will not affect on website ranking.
Listing of subdomain and root domain will be treated as same, this means that the subdomain will have much effect on site diversity
Now you have to keep in mind that you cannot list more than two pages on the same site if you have to list the subdomain then you will have to register by visiting another website if you already have a root domain on that website, if you have listed pages, you cannot place further listings on that website.
The broad core update launched in June 2019 but site diversity is not part of it,
Will not affect all search results in Diversity Change
In June 2019, Google has launched a site diversity change and has launched Broad Core Update.