The function in ./core/modules/dblog/dblog.admin.inc has been replaced with dblog_format_message()
The dblog_theme() implementation has been deleted.

Drupal 7 code:

theme_dblog_message(array('event' => $message, 'link' => FALSE))

Backdrop code:

dblog_format_message($message, FALSE)
Introduced in branch: 
1.0.x
Introduced in version: 
1.0.0
Impacts: 
Module developers
Theme developers
Related Github Issues: 
https://github.com/backdrop/backdrop-issues/issues/539