Photos smart album no longer on iMac

Are you asking about Shared Albums, not Smart albums?

Or have the smart albums been synced manually from your Mac to your iPhone and iPad and are now appearing in the "From my Mac" section of the list of albums?

Synced albums will remain on the mobile devices, even if you delete them from your Mac, until you sync again. Then they will be replaced by the albums you select for the next sync. If the photos in the albums on your iPhone and iPad are now the only copies you have, and they are in synced albums in "From my Mac", do not sync again or enable iCloud Photos, until you have saved the photos, or you will lose these photos.

To save them in the Photos Library on the iPhone or iPad duplicate the photos. The duplicates will be saved to the Camera Roll and can be downloaded again from the device.

Hard to say. Smart albums aren't supposed to show up on iPads or iPhones. Normal albums that appear on your iPhone(s)/iPad(s) should propagate to your Mac(s) connected to the same iCloud Photos Library (with the same Apple ID) over time.