GTK 3.20 and GTK 3.18 are now merged into a single GTK 3.0 directory. This will reduce work needed in the future for maintenance and updates.
In the past, GTK3 had an extremely unstable theming API, however, as GTK4 draws near, less focus has been given to breaking existing CSS with GTK3. This makes serving specific versions of GTK CSS redundant and doubles the work required to make changes for gains most users aren't likely to see.