I’m also puzzled about the -d option - I don’t see it in mitmdump version 4.0.4 either. Ignoring it seems to work.
But I’m thrilled to see that by adding -s, and using mitmproxy/har_dump.py I can convert a previous dump file to HAR format, which seems to be one of the standard ways to share HTTP traffic:
mitmdump -n -r dump.mitm -s har_dump.py --set hardump=./dump.har
See also First class support for HAR · Issue #1477 · mitmproxy/mitmproxy