how can i specify in my migration not to migrate unpublished content? in a drupal 8 migration, is there a way to specify in the .yml that i dont want to migrate any content that is unpublished .  Or would that have to be done in a plugin?
You could try using the skip on value plugin.
Untested, but probably like:
process:
  status:
    plugin: skip_on_value
    method: row
    not_equals: true
    value: 1
    source: status

Should only import published content.