In the olden days of Android phones, there were more than just three navigation keys. Some devices had a dedicated search key, and most phones had a menu button that acted like a system-wide right-click key. However, the menu key has been deprecated for years, and it has finally been laid to rest with Android 10.
The menu button has technically been deprecated since Android 3.0 Honeycomb was released, when Google introduced the ActionBar feature for developers (which made it easy to put menu buttons in the app itself). However, the navbar menu icon continued to appear for apps still targeting Android 2.3 Gingerbread or earlier.
A bug report was filed a few months ago for Android 10, where someone noticed the menu button no longer appeared for legacy apps. Google recently closed the bug, stating, "this was intentionally removed as the API has been deprecated for several releases now."
While nearly 9 years is very generous as far as deprecation dates go (starting from the release of the Honeycomb SDK), this change will break the small amount of apps and games that haven't been updated to move the menu icon in-app. If this change affects you, it's probably time to find alternative apps.
- Google Issue Tracker