Use case for this in core is the "Number of items to index during cron runs" setting in admin/config/search/settings:

image

It currently allows for a choice between 6 pre-defined values, as shown in the screenshot above. Ideally, we could allow any custom value (grater than 0), and offer these values as suggestions, instead of having them hard-coded.

The HTML5 number field allows that as follows:

<input id="ticketNum" type="number" name="ticketNum" list="defaultNumbers">
<span class="validity"></span>

<datalist id="defaultNumbers">
  <option value="10045678">
  <option value="103421">
  <option value="11111111">
  <option value="12345678">
  <option value="12999922">
</datalist>

Which makes the <input> field a free-type number field + offers the suggested values in a drop-down in the same field:

image

GitHub Link: 
https://github.com/backdrop/backdrop-issues/issues/5285
GitHub Issue #: 
5285