The link_default field widget (core/modules/link/src/Plugin/Field/FieldWidget/LinkWidget.php) now correctly saves link attributes from the form element of '#type' => 'value'.
Previously, LinkWidget::massageFormValues() completely ignored this value and always set the link options to an empty array.
Care was taken to make this change work with as many known contributed modules as possible. However, solutions for implementing link attribute widgets are varied and not all widgets will be compatible.
Action to take
Maintainers of a widget extending LinkWidget that uses link attributes should test that widget to ensure that this change does not disrupt that functionality.
Site builders who use attribute-setting modules should test their content to ensure that those attributes continue to work as expected.
