Does a land with a subtype like Plains still count as a basic land if it doesn't have the basic supertype?

The rules distinguish between a land's supertype and its subtype. The supertype 'Basic' is what makes a land a basic land, as defined in CR 305.6. A land with a subtype like Plains, Island, Swamp, Mountain, or Forest is simply a land with that subtype — it is not automatically a basic land unless it also carries the Basic supertype.

CR 205.4 defines supertypes (Basic, Legendary, Snow, World) as characteristics separate from subtypes. CR 305.6 explicitly states that the basic land types are Plains, Island, Swamp, Mountain, and Forest, and that any land with one of these subtypes has the corresponding mana ability — but this does not grant the Basic supertype or make the land a 'basic land' for rules purposes.

This distinction matters in several game situations. For example, effects that say 'search your library for a basic land card' (like Rampant Growth) cannot find a non-basic land even if it has the Plains subtype. Similarly, the 'four-of' deck-building restriction (CR 100.2a) does not grant an exception to non-basic lands with basic land subtypes — only lands with the Basic supertype are exempt from that limit.

Concrete example: Stomping Ground is a Mountain Forest (it has both subtypes and thus can tap for red or green mana), but it lacks the Basic supertype. A player casting Rampant Growth cannot search for Stomping Ground, and you may only run four copies of it in a Constructed deck.

In summary: the subtype grants the mana ability (CR 305.6), but only the Basic supertype makes a land a 'basic land' for all other rules and card effects.