The add_to_log() function is being replaced by event triggering.
While all add_to_log() calls in core are being replaced, the function should be marked as deprecated so that messages are shown to developers of add-ons to prompt them to upgrade. It would be good to direct them to transition documentation in such messages. This might also help discover add_to_log() calls we might have missed in core.
This issue represents the first stage of deprecation. A period of deprecation should be decided (default is two versions, but this may need longer) and a second issue should be created to finalise the deprecation. See http://docs.moodle.org/dev/Deprecation#Moodle_Core_deprecation_process