Adobe recently sent out an email to all Creative Cloud customers regarding changes to version download availability. (If you did not receive this email you can read it here. If the page is blank please try reloading it.)
See also the statement here on their website regarding this:
Changes to Creative Cloud Download Availability
What this means for our customers
Previously Adobe has offered older versions of their software for download via the Creative Cloud application. The availability for which of these older versions can be downloaded has now changed. As of May 9th, 2019 only the absolute latest release (CC2019) and next latest release (CC2018) will be available for download. In the past this has extended back to CS6 and included CC, CC2014, CC2015 and CC2017.
If you are using CC2015 or CC2017
Downloads for these versions are no longer available via the Creative Cloud application or for packaging. If you already have CC2015 or CC2017 installed then you are not impacted. However it is very important that you do not uninstall these versions as they can be problematic to reinstall. Please see the pages here if you need to install: CC2015 | CC2017
Because of this change in policy from Adobe we encourage our users to consider updating to CC2018 as soon as possible to avoid potential installation issues. From September of this year we will no longer support CC2015 and discourage users from using CC2017 in the future following Adobe's announcement. Note that CC2015 and CC2017, according to Adobe's email, are still authorized versions of InCopy and InDesign and will continue to work as normal and do not need to be uninstalled.
If you are using CC2018
If you are using CC2018 already you are not currently impacted by this change as it is still available for download as normal. However, this may change in the future when Adobe releases a new version of Creative Cloud.
We are working on plugins for compatibility with CC2019 and have plans to have them available in June. An update for Transfer Manager will also be made available at that time and will be needed to work with CC2019.