Part of Lightning Migration work, we found that "task and Event subject line doesn't contain picklist "combo box" - only plain text field" and this a known issue

https://success.salesforce.com/issues_view?id=a1p3A000000IY4uQAG 

We do have an engagement with salesforce services team and I did consult with Salesforce Solution Architect about this known issue and he did provide me below workaround.

https://help.salesforce.com/articleView?id=How-to-use-a-preset-Subject-field-for-Events-and-Tasks-in-Salesforce1&language=en_US&type=1

We tried to implement this functionality. It works fine but still, there are some of our observations on this.

1. The new subject picklist field is available only for Activity Time line (New Task or New Event Actions), but not on Open Activities related list (New Task or New Event Buttons). So when users try to create a new task /event from Open Activities related list, still the layout will show the standard text field.

2. New Task and New Event actions in Activity Timeline will be available only for particular record type we configure at the global action, so it will not allow the user to select Record type while creating Task/Event.

3. The new pick list subject field will be available only in the Create Task/Event layout. But when the user tries to edit a task/event, the standard text field only will be available.

For your information for those who have not tried yet also if anyone overcome these problems do let me know. Thanks