The feature is there.
- Assignment setup > Submission Settings > Attempts reopened to manually, then
- Maximum attempts to some value. I usually set 3 as that's our resit policy.
Now when you grade the assessment, there is an option at the bottom of the grading screen to allow another attempt.
- Choose Yes and save.
To view different attempts as a grader, there is now a new "View a different attempt" link at the bottom of the grading screen. It'll also show you the attempt number.
All of the attachments, feedback, etc for each attempt are isolated from each other which is great for keeping evidence of improvement.
The learner sees the feedback on their assessment screen, which does, unfortunately make for a very long scroll (this is fixable if you know how to theme moodle).
When the learner goes to submit extra attempts, they can choose to use the old submission as a template, copying the existing attachments, text, etc or submit from scratch. The previous attempts remain untouched.
Hope this helps!