A dropbutton widget was added to Drupal 8 (2012).
Later it was extended to support dropbutton variants (2019).
And some recent work to use it in the View UI (2025).
Dropbutton has issues with screen readers (2013).
An issue was created to replace Dropbutton with Splitbutton (2013).
It sounds like it will be added instead of upgraded in place to prevent breaking changes in themes.
It is similar to the split button in bootstrap. The label can be clicked, and icon expands the list.
Starterkit theme has some css.
Drupal core css and js is here. It still depends on jQuery.