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The backups are retained for the same period as the purgeartifacts in purge settings.
Exactly 1 backup is retained for each ProjectID/ProjectVersionID/EnvironmentID. If you deploy again, the version which was replaced we become the back-out version.
To view or download a backup, go to the deployment that had a backup. Click Artifacts. The backups are shown below the artifacts.
To restore the backed-up files, you can create a deploy input that replaces the artifacts with the backups and deploys.
You could also source control the backed up files and then build and deploy them manually.
If FlexDeploy has deployed another older version of the files, you could also just redeploy that older version.