I can't think of a precedent, a time when a platform vendor deliberately broke so much application software. It's possible that it's happened and I don't remember, but I find it hard to believe that I would not have heard of it. So much app breakage would surely make a loud sound!
Some of my apps will die tomorrow. For example, FriendsOfDave and dwcodeupdates. It probably won't be worth it to convert these little apps to use OAuth. It will impact very few people, mostly me. (FOD is like a quick aggregator for people whose updates I never want to miss. Codeupdates helps me confirm that updates made it through the OPML Editor's code management system. Neither are mission-critical, just nice-to-have.)