git annex dropunused is simple enough.
git annex dropunused
But how do I perform the equivalent procedure on an rsync remote? I'd presume that git annex copy --to remote is not sufficient.
git annex copy --to remote
Well, in your defense it's documented there because I can't think of a better place to put it. Maybe you looked in some other places first before finding it there, and I should add a pointer to those places?
(For the curious, the magic run is just: git annex unused --from remote)
git annex command --help
git annex dropunused --help
You might be using an older version. Here's the output of that command in the current version:
Usage: git-annex dropunused NUM|RANGE ... [option ...]
-f REMOTE --from=REMOTE drop content from a remote
To see additional options common to all commands, run: git annex help options