I think there should be a category for the game that has the same rules as the "Default Settings" category save for an additional rule in which no biters can be killed. This would likely create a change in strategies needed to complete the run, as dealing with pollution becomes more important, as well as resource efficiency and locating resources that aren't near biter nests.

has anyone ever completed a game on default without killing a single biter in any time period?

We only add Categories to the leaderboards that are already established in the community. I encourage you to run anything you are interested in and post your runs in the forums or the discord. Maybe others will like your inspiration and pick it up. I run a lot of non made challange categories or runs myself, without the intention of ever adding them to the leaderboard.