The Simple XML Sitemap module was originally created by gbyte as a temporary replacement for the non-functioning Drupal 8 XML Sitemamp Module. After putting some more work into it however, we decided to keep using it in our D8 & D9 projects, as it is very lightweight, simple to use and most importandly, adheres to a newer XML sitemap standard.
The 2.x branch features most of the functionality of the heavier XML Sitemap module while also featuring hreflang and image XML sitemaps, which is a new Google standard for creating multilingual XML sitemaps that should improve SEO even more.
The 3.x branch adds a robust generation process that works with huge sites in limited environments. It also introduces the concept of sitemap variants which are instances of different sitemap types which in turn are made of sitemap and URL generators. This makes it possible to run several different sitemap types on one Drupal instance.
The 4.x branch is a rewrite with developers/integrators in mind and makes much greater use of Drupal's entity API dropping some of its very specific chaining API. On top of that, it features many code and UI improvements.
Here is the description from the module page: