How do you escape @ in markdown?

This isn't a Markdown feature. It appears to be a Dendron feature called User Tags:

Dendron allows you to tag users in your notes. You can tag users inline like @example. They are similar to tags, except that they are meant to reference users. In a vault shared with many people, where each person can have their own user note. Or you can also use it yourself to keep track of information about people you know.

Playing with this a bit I don't see a good way to escape a particular user tag. Backslashes, zero-width spaces, etc. don't seem to prevent this from being linked.

Depending on the context, it might be appropriate to mark this as inline code with backticks, as you have done in your question: @Input.

Alternatively, if you don't intend to use User Tags at all you can disable them:

Don't like user tags? You can disable them by setting the enableUserTags configuration to false.

This can be done by editing the dendron.yml file in the root of your workspace, or via the "Configure (yaml)" command.