For anyone too lazy to search for ss_log.php (which is actually log.php) to read the class notes before searching the forums like me, the now depreciated code

Debug::log_errors_to("/my/logfile/path");

can be replaced with

SS_Log::add_writer(new SS_LogFileWriter(Director::baseFolder() . '/error_log.txt'), SS_Log::ERR);
SS_Log::add_writer(new SS_LogFileWriter(Director::baseFolder() . '/error_log.txt'), SS_Log::NOTICE);
SS_Log::add_writer(new SS_LogFileWriter(Director::baseFolder() . '/error_log.txt'), SS_Log::WARN);

Saves you a couple of minutes.