Is there a way to enable this for deck attachments or is it a planned feature?
You should open a new issue for this in the issue tracker, so the maintainer will get notified: https://github.com/nextcloud/deck/issues/new?template=Feature_request.md
Side note: The Deck Android app does support preview thumbnails for attached images.
Update: This is available for attachments which have been uploaded with Deck 1.3.0 or higher. A migration step for earlier uploaded attachments is supposed to be provided in a future release.
The current version of the Deck Android app does also support this.
More information here: Inline preview and popup of Attached Images. · Issue #972 · nextcloud/deck · GitHub