"Undeleted" means deleted and then restored. "Non deleted" means that have never been deleted at all. The sentence you gave seems to be correct English, although it's obviously written by an engineer, and not a humanities graduate.

Technically, it is a proper sentence. Without context it is hard to know if it makes sense. Perhaps it would be better stated: There exist tasks that have not been deleted.

It's rather awkward. I use "There are un-deleted tasks." However, be sure to recognize that "undeleted" or "un-deleted" are "IT words" and are not actually proper English.