Give dropdown more affordance to differentiate single/multiple dropdown

Background

We have a feature which has multiple dropdowns, and it is a mix of single and multiple selections. Like below

With the new update, we removed the "checkbox" and "radio" button affordance, but it is not easy for the user to figure out what is a single selection and what is multiple. In the solution validation research, we found out that 6 out of 6 participants, none of them can tell which one is single selection, which is multiple. The only way they can do it is to try to select multiple or single and then realise that this is single or multiple. In my opinion, this is not a good experience, because

• User can only select by trying out; with the number of dropdowns in the setting, trying out by one could be annoying
• Next time, the user forgets what is multi-selection, and what is single-selection, they need to try out again.
• If a user never tried and assumes everything is a single selection, they will never find out some of them are multi-selection by design. They will run into a situation which makes the configuration difficult.

Please see Dovetail insight for details

Solution

Based on the previous Slack discussion, there are also some technical concerns, I will prefer to consult @jeldergl for more ideas exploration. So far I think bring checkbox-ish affordance for multi-selection is a good idea.

Note: the new design is plan to develop in next 1-3 milestones, preferably we can have the update of design for the dropdown before that