Ok, got it. Maybe this could be a feature request or a plugin then. I bet many would find this very convenient to cross out completed tasks in their checklists.
I’m having trouble finding an answer on that, but this css snippet makes the checked task less visible. (leaving that “line-though” part in, hoping one day it starts working)
.markdown-preview-view ul > li.task-list-item.is-checked {
opacity: 40%;
text-decoration: line-through;
}