The only less-old stuff added to the legacy content is the HiveWire3D figures. Dusk, Dawn and Baby Luna. As Donnen says. Not worth the upgrade for content alone. There are some P12 features that would make it worth the price. For me the support of more modern GPUs is a big deal. P12 Supports RTX 30 series. And has support for Optix rendering. That gives a massive render speed boost. It also adds the OIDN denoise which can further reduce rendering times. To me those time saving features are a big deal.

If you have a new Nvidia GPU, especially an RTX 20 or 30 series it's worth it just for the 10x faster renders.

P.S. Almost forgot adaptive sampling. Allows you to set high render settings, basically over rendering, but it will stop rendering when it reaches the quality you set for it. Another time saver.