If you’re still clinging to your Windows 8.1-powered PC or tablet, you’re going to have add the official Dropbox app to the list of apps you can’t use with your device. The app can no longer be installed from the Store and it’s no longer running if you try and open it. What is still working is the desktop program. You can still download that and sync your files there. Microsoft has already dropped its support for Windows 8.1 six months ago, so it doesn’t come as a surprise that companies are moving on from it as well.
Source: OnMSFT