This issue has been fixed in the 7.0.1 update to Lightroom Classic CC released on October 26, 2017. Now, the Remove Old Versions option in the Creative Cloud dekstop app is disabled by default for users upgrading to Lightroom Classic. See this blog post for details.
If you uninstalled Lightroom 6 accidentally while updating to Lightroom Classic, skip to the Reinstall Lightroom 6 section.
While updating to Lightroom Classic from previous versions using the Creative Cloud desktop app—for instance, while upgrading from Lightroom 6 to Lightroom Classic—the previous versions of Lightroom are uninstalled by default.
By default, all previous versions of apps are uninstalled when you update to the latest version using the Creative Cloud desktop app. If you want to retain the previously-installed versions of the apps, you must change the default update settings in the Creative Cloud desktop app.
If you choose to switch back to Lightroom 6, note that any changes or edits made in Lightroom Classic are not compatible with Lightroom 6.
If you already uninstalled Lightroom 6 accidentally while updating to Lightroom Classic, follow these steps to install it again: