There's no (official) way to enable deleting from command-line with the /keepuptodate.

But you can use scripting instead:
https://winscp.net/eng/docs/scriptcommand_keepuptodate
The keepuptodate command has -delete switch.

hi.
I'm trying to keep an FTP site updated with files from my local directory, but I'm not sure how to make sure that deleting a file from the local directory also removes it from the remote FTP site.

thanks,
Martin