IndexNow support added
After Microsoft decided to silently kill the sitemap ping API and instead use the new indexNow protocol, I figured the search engines submodule should support it.
4.1.x adds support for IndexNow, converts the existing Bing search engine entity to use it and adds new entities (Yandex and IndexNow).
If you had the simple_sitemap_engines submodule submit the default XML sitemap to Bing, the update hooks will make sure to enable submitting the corresponding entities to IndexNow on every entity form save (don't worry, given the right permission, this can be overridden before submitting the form).
You will have to generate a new verification key on your production sites - just head to the status page (admin/reports/status), or directly to admin/config/search/simplesitemap/engines/settings for guidance.
For more information on this release as well as on IndexNow, see this post.