Allowed HTML tags in "Allowed value list" are not working
According to your question comments, you are trying to create a form with an options list element using the core select element widget.

This widget is intended for creating select lists. Markup within select list options is considered invalid HTML. That's why the select options widget has a sanitizing function for its option labels, namely OptionsSelectWidget::sanitizeLabel(). This sanitizer is used on each widget settings form submit and cleans your labels from any HTML.

If you really require an options element with HTML in the option labels, you have three choices:

(1) Choose another widget type for the list field that supports HTML markup natively. Obviously you choose the select widget. Choosing checkboxes or radios as widget type in the "manage form view" tab will use the less restrictive satitizing function of the the abstract OptionsWidgetBase class.

(2) If you need an entirely unfiltered list, create a custom field widget for your list field. Make sure to also implement a form element different from select that supports HTML in its labels.

(3) Leave the sanitized labels in the Core widget configuration as is and use hook_field_widget_form_alter() to add your markup to the labels of the element you intend to replace the Core elements with.