In Photoshop 2019 it's possible to set guides to 50% of the canvas; I tried to do the same in Ai but it didn't work: i had to calculate the distance in points, and then set it manually with move guides (ctrl+shift+m). Is working with percentages of the artboard possible in Illustrator 2019?
Yes and no, well ... kind of.
Make sure the guides are unlocked, and drop a guide on the artboard. Then, if you type into the X or Y field in the options along the top like this "210*50%", where 210 is the size of the artboard, then hit enter, Illustrator will do the maths inside the entry field, and move the guide to that position.