Relative adjustments are not a LR feature
Not strictly true. Through Library Module > Quick Develop relative adjustments can be made to WB - Temperature and Tint separately, Exposure, Clarity and Vibrance in small or large jumps. For instance, Exposure can be altered by +/- 0.33 stops or full stops and Clarity by 5 or 20 units. Select a group of photos in the grid and the same amount of change will be applied to all of them, regardless of their individual starting points.
You are right - since I am seldom using the Quick Develop I may sometimes forget about it. However, in the context of the question and the develop module as such, relative adjustments are not available in LR.
it's not precisely what i was searching but definitely better than going through 200 pics one by one and making the same change