This past summer assisted a school district with a very large implementation of Moodle 1.9. Their decision was to install a fresh 2.x and restore 1.9 backups. Luckily, for them, their version of 2.3 was 2.3.2+. Through installing the latest and greatest version 2.3.x and comparing sites, the latest 2.3.x wouldn't allow the 'work around' for deliverance from legacy.
Simply turning off legacy from appearing in site/course administration doesn't actually go into the new file system and 'straighten out' those files tagged as 'legacy'.
Suggest trying that with one course, run a backup of that course, then attempt to restore it to the same site. Think you'll find the dreaded 'legacy' re-appears.
New courses do not automagically use Legacy even if turned on.
Of course, this was tested this past summer. Things may have changed since then (seems to be the order of the day ... one step forward, one step back sometimes :\).
'spirit of sharing', Ken