Background

Users from other sync tools expect rclone sync to:

1. Sync bidirectionally, mirroring changes in both directions
2. Preserve conflicts, abort, or require interactive resolution

While documented as destructive, this expectation is reasonable -- bidirectional sync with conflict handling is standard in similar tools, including rsync's default behavior -- and will cause data loss if acted upon.

Solution

Similar to rsync -av empty:/ ~/CriticalData/ without --delete, rclone sync empty:/ ~/CriticalData can skip all delete operations and print a warning instead. It would still overwrite matching files as rsync would, as storing conflicts would seem a bit out-of-scope, but it would save users from a common footgun with minimal changes.

This would be a breaking change, but the migration would be smooth: users would notice accumulating files, read the log, and follow its instruction to add --delete, returning the dest to its expected state on rerun.