How do I migrate attached files from AWS S3 back to the local filesystem to retire an S3 bucket?

Use the file CLI: php manage.php file migrate --backend=3 --to=D. Alternatively, dump the entire S3 bucket via the AWS CLI, move the files to your filesystem folder, and point the plugin instance at that folder. osTicket uses the same folder structure on S3 as on the filesystem (files stored in a subfolder matching their first character, e.g. key 'xHSb...' goes in 'x/'). Note: Linux filesystems are case-sensitive but Windows is not, so take care with key casing.