Previously it was possible to install older versions of Adobe software via the CC-app. In Photoshop, for example, back to the version CS6. This is no longer possible because Adobe provides a supporting older CC versions.
Laut einer Meldung auf der Adobe-Webseite sind nun bestimmte ältere Versionen innerhalb der CC als „nicht mehr für den Gebrauch autorisiert“ gekennzeichnet (Link). Although this report refers to "Creative Cloud for enterprise", so for businesses, but when I check in my own CC app, then there are also - unlike until recently - the older versions are no longer available. Photoshop is the oldest existing version is now CC in 2018 with their last bugfix update (v19.1.8). Now this version is indeed solid and stable, but the change could cause problems for users of older operating systems, as well as extensions or plug-ins with him, which still require an older version of Photoshop.
More problematic than for us image editor that can open ancient Photoshop files with the latest version of Photoshop problems, this policy change from Adobe but is probably mainly for InDesign users who thereby not entirely unproblematic IDML exchange format for their layout files rely, instead of opening the InDesign files with the respective older version of the program.
Users of the latest purchase version (CS6) should be not affected by this change, only is this and several subsequent versions just no longer as before the users of the CC subscription available.
Adobe stellt die Unterstützung älterer CC-Programmversionen ein: Über die Hintergründe dieser Änderung äußerte sich Adobe bislang nicht.