McDonald's app, verification e-mails don't work
I was having a problem seemingly similar to:
McDonalds app won’t verify on iPad - Apple Community
I was suspecting that the link from the e-mail wasn't working due to privacy or tracking, so I turned those off, to no effect. It turns out the answer is simpler: Safari was set to request the Desktop version of mcdonalds.com. Change this to request the mobile version, then the website will properly prompt to open the app.