Hide implementation detail past_viewer_recommendations from ViewportBlueprintArchetype

[Wumpf]

We should split internal implementation details like this out into a separate unlisted archetype (maybe another sub-archetype?) -- there's no reason for a user to be aware of this.

Originally posted by @jleibs in #5419 (comment)

The concern is that the past_viewer_recommendations field on ViewportBlueprint archetype is confusing to users.
Note that as of writing there's some disagreement both on the severity of that and the strategy of how to deal with this.