could a mod make a damage percent category where you have to take any type and amount of damage the fastest.

Given you can self-damage, that category with the stated requirements wouldn't work.

A. We do no take text suggestions for categories. https://www.speedrun.com/ultrakill_category_extensions/forums/hrobz

B. No. This would just be Death% (a category we already had but has been archived due to already being maxed and boring) but even more boring.