GTK, frankly, has been going downhill when it comes to customization (including, but not limited to, themeing) for at least a decade now. Many GTK themes out there today do not support GTK4 (the page you linked to is confusing; even though it's the "GTK3/4 Themes" category, many don't support GTK4).
GTK has more and more become a stripped-down toolkit that requires you to write or use a "platform library" (like libadwaita) in order to do useful things. Judging by the deprecations in GTK4, as well as statements from GTK developers, GTK5 will be even more stripped-down. This just makes it harder for non-GNOME projects to use it.
On top of that, each major GTK release comes with drastic changes to how classes of widgets work, which for smaller teams could mean years of work to migrate to the new version. I don't begrudge the GTK developers their ability to make all of these sorts of changes; after all, I'm not paying them for any kind of support or feature set. But it's still frustrating for smaller teams that don't realistically have the ability to take on the maintenance burden of an entire UI toolkit.
GTK has more and more become a stripped-down toolkit that requires you to write or use a "platform library" (like libadwaita) in order to do useful things. Judging by the deprecations in GTK4, as well as statements from GTK developers, GTK5 will be even more stripped-down. This just makes it harder for non-GNOME projects to use it.
On top of that, each major GTK release comes with drastic changes to how classes of widgets work, which for smaller teams could mean years of work to migrate to the new version. I don't begrudge the GTK developers their ability to make all of these sorts of changes; after all, I'm not paying them for any kind of support or feature set. But it's still frustrating for smaller teams that don't realistically have the ability to take on the maintenance burden of an entire UI toolkit.