Like Rick Heijnen said it should be optional in settings.
I use a separated Google calendar for tasks to be able to plan them to a specific time at that day.
After completion it stays like a diary of done tasks.

I too would like to see an options where the user can set if the tasks linked to a calendar view should be deleted from the calendar view once completed. This requested feature has been under review for long and hopefully can be implemented soon.

I would like the task to be hidden so I can see remaining tasks only, but I agree that this should be managed using a system setting for those that want it to remain. If it remains on the calendar and you don't finish it, how will you know (other than looking at UTL)?

I use the calendar directly for events, not the task list. In my opinion, the task-calendar integration is helpful for not for events, but for task deadlines that need to be visualized on the calendar.
And if I complete a task, the event in the calendar can be deleted too. This is especially useful if the deadline is still in the future so it doesn't crowd up my calendar unnecessarily.

I can see why some people like their tasks to stay on the calendar, so I suggest to make deleting tasks an option.

I personally like my tasks to stay on my calendar after completion so I can look back later and see what I did on any given day.

If a feature like this is implemented I'd like it to be optional

Why should it delete from the calendar? The calendar doesn't delete meetings you have entered and attended?

... completed IT stays... sorry, typo