My most hated one is not allowing me to right-click to open a context menu, or even worse, making right-click act the same as left-click, redirecting me to some page I never intended to go to.
Likewise usurping Ctrl-F with their own shitty global search function rather than the perfectly good page search provided by the browser. Looking at you, Confluence.
Usually Ctrl-F still works for me, but / almost always gets consumed by the stupid search that never works properly. But then that's usually because you're on a site with infinite scroll so even the Ctrl-F search is going to be limited, even if you are in the correct thread.
One thing is really dislike about modern UI design is where you see a headline that seems kinda interesting but you don't click on it right away. Come back a bit later and it's just gone. There was some A/B testing on the headline and now it's completely different, probably now all clickbaity so I won't be inclined to check it out.