After six months of work I'm delighted to tag the first stable release of the 4.x branch of the (Not-so-) Simple XML Sitemap module.
The project is in a really good place right now. At the moment of writing, drupal.org reports it being actively used on around 90k of Drupal 8/9 websites while having 0 open bug reports. This either means you people are lousy bug reporters, or we are doing a descent job at responding. :)
Module rewrite with developers/integrators in mind
4.x makes much greater use of Drupal's entity API dropping some of its very specific chaining API. Feel free to take a look at the roadmap ticket for specifics.
New UI
We now have a much nicer UI for creating, editing and sorting sitemaps as well as sitemap types.
API usage
In a nutshell, sitemap variants are now sitemap entities. These are of a sitemap type (sitemap type entity) that is defined by URL generator plugins as well as sitemap generator plugins.