Yes, we could do that in Trello and I really miss this feature in SmartSuite. Now we are deleting old checklist items manually and it's time consuming and inconvenient.

I agree. I'm using checklists as a little task manager within a record, and the more items I add and then cross off, the longer the task list becomes, eventually dominating the whole record. It would be great if completed tasks filtered into a completed section, kind of like how My Work has separate tabs for Open and Closed Assignments