When you run the "Update" to Acrobat DC, Acrobat XI, and any plugins you are using, disappear. There is no warning. Pro XI is just simply gone.
It is not possible to install Pro XI along side of DC. If you try, the install seems to work, but the Applications folder only contains DC.
Also: Acrobat Pro XI does not appear in the Previous Versions list in the Creative Cloud app.
The only way I have found to get Pro XI back is to uninstall DC, then go to the Pro XI Download page: https://creative.adobe.com/products/acrobat
Click Download, and the CC applet will download and re-install Acrobat Pro XI. You will then need to re-install all your plugins.
Adobe: Thank you so much for creating a massive support headache for your customers! Your ability to screw us over knows no bounds.