The page describes a feature for a previous version of DokuWiki and if relevant will include a link to a current feature (use the backlinks feature to get an overview).
See also the schedule for deprecation on the release page.
Since 2014, the function
dbg_deprecated($alternative = '') is used for logging calls to deprecated functions. It write to the
deprecated log shown in the LogViewer (default on) when
deprecated is unchecked in dontlog configuration setting. (Up to Hogfather 2020 release: When the config allowdebug is enabled, you can find logged calls in the debuglog.)