No paywalls, no "premium" versions. However, you will not find these on the modern Simtropolis STEX (which now uses a different system). Instead, you have two primary sources:
Even with these collections, some items require "dependency packs" (common texture or prop files) to display correctly. If you see a "brown box" in-game, you are missing a dependency listed in that item's readme file. Modern Alternative: sc4pac How to install content from STEX Discs - SC4 - Simtropolis stex collection vol 1 2 3 for simcity 4 free install
Search for "Simtropolis Exchange" or "STEX" on the site. While specific "Volume" installer .exe files are rare now due to security protocols on modern computers (which block old installers), the community now recommends downloading or specific "Landmark Sets." No paywalls, no "premium" versions
Use SC4DependencyTracker (old but helpful) or search the lot’s filename on SimTropolis to see required props. For most casual players, expect some lots to be broken unless you hunt down packs. If you see a "brown box" in-game, you
These represent a snapshot of the exchange as it existed up until 2013, featuring even more complex assets and refined modding techniques. "Free Install" and Availability