Updating to CC 2017 has uninstalled my 2015 apps.
Yes, that's the crazy default setting that Adobe has adopted for CC 2017. Uninstall previous versions unless you tell it not to. I have no idea why Adobe thought that was a good design decision.
I had plug-ins that only work on InDesignCC 2015.
Can you post a screenshot showing what options are in your Cloud Desktop app?
Have you tried
"Click FIND ADDITIONAL APPS. Then click All Apps and choose VIEW PREVIOUS VERSIONS at the bottom of the drop-down. Click Install for the program you want to use, and choose the version you want to install."
Here's the screen-shot. The VIEW PREVIOUS VERSIONS is not an option. I read in another forum that that option is not available after OS 10.8; I'm running 10.10.5. Very strange.
The Plug-in is HP's SmartStream Designer for use in conjunction with our Indigo 7r (a pretty cool plug-in, btw). It has a lot of variable data functions we use almost every day. As you might imagine, not being able to offer these features to our customers really hurts our bottom line.
The adobe id currently signed up in InDesign is a trial id and not a subscription id, Hence you are not able to view the option. I am currently signed up with a subscription id and the option is visible as present in screen shot below. Please sign in Thor using subscription id to install InDesign CC 2015.