Oh yes gods slack in particular got a lot worse with the most recent update, but the site I think takes the cake is one we Canadians use for food delivery - skipthedishes.com . Gods this site breaks so many things, options to update things are hidden among menus that switch the current page while not being ctrl-clickable into a new tab (which can trash your current in-cart order) and the whole site loves "minimalism" i.e. let's put soft grey text on a white background to drive contrast into the ground.

Given that this service's value proposition is basically "As a company we have a highly usable website and a fleet of delivery drivers" the fact that their website is trash is super annoying.

(The site is blocked behind address entry, if you'd like to try it out may I suggest 1 West Georgia Street Vancouver BC)