Asynchronous backups and restores have been an option since Moodle 3.3, and are a good improvement over the synchronous version.

One of the main reason is that they do not block other actions.

It is now time to remove synchronous backup and restore when backing up and restoring a course from the UI. This follows on from 4.5 when they made on for all installs and in 4.4 when they made on for new installs.

This won't change the underlying backup and restore functionality or availble API's, as it is used in several places across LMS.