@PKirova here’s an idea submitted to HubSpot that asks for tasks to be deleted, so that tells me that no, tasks between HS/SFDC are not deleted. This would also make me believe that if a task is deleted in SFDC but exist in HS, it very well may create them again in SFDC. I’d test that for sure.
In the ideas forum, give that idea an upvote and comment. The more popular ideas get the attention of the HubSpot product team.