After an upgrade to 2018-04-22c "Greebo" (using the wiki upgrade plugin), the file dokuwiki/inc/Plugin.php was missing. Retrieving the previous copy of this file restored dokuwiki operation. The upgrade plugin displayed the following final 3 text status lines:
All files updated.
Deprecated inc/plugin.php deleted.
Upgrade completed. Enjoy your new DokuWiki
The host is running macOS with a file system that is setup as case-insensitive (the default). This means the following somewhat ambiguous file accesses are possible for a file named inc/Plugin.php:
$ md5 dokuwiki/inc/*lugin.php
MD5 (dokuwiki/inc/Plugin.php) = b1fe14074f08a3c3d3d7bd82a2b4178a
$ md5 dokuwiki/inc/plugin.php
MD5 (dokuwiki/inc/plugin.php) = b1fe14074f08a3c3d3d7bd82a2b4178a
$ md5 dokuwiki/inc/Plugin.php
MD5 (dokuwiki/inc/Plugin.php) = b1fe14074f08a3c3d3d7bd82a2b4178a
$ ls -al dokuwiki/inc/*lugin.php
-rw-r--r--@ 1 _www staff 251 6 Jan 2019 dokuwiki/inc/Plugin.php
i.e. deletion of plugin.php will always remove any Plugin.php because filenames are case insensitive.
Is it possible that the upgrade attempted to delete file inc/plugin.php but removed inc/Plugin.php instead?