From an email:
Improved Texture Standards
Our textures are higher quality than they were 9 years ago, but our format changes have lead to an inconsistent user experience.
So to increase consistency, we’re announcing new texture standards: a framework that sets expectations for what you’ll get when you download textures.
In a nutshell, here's the main changes:
Defined PBR Maps: A predictable set of deliverables: Base Color, Roughness, Normal, Displacement, Ambient Occlusion, Metallic and a new ORM map for realtime.
Improved Filenames: All files are named with the new structure of: Poliigon_DescriptiveName_AssetIDNumber_MapName (no more confusing _metalness at the end of every file!)
More Resolutions: 8K-1K for large, 8K-.256 for small.
Set Physical Sizes: All surfaces are 2.5x2.5m (large surfaces) or 30x30cm (small surfaces)
Material X - An interoperable industry compatible .mtlx file that can be natively imported into multiple softwares.
From today all new textures will meet these new standards, including the latest batch of 13. Our goal is to remaster the entire texture library to these standards by the end of 2025. In certain circumstances, a texture may be unpublished if the source material is too old, but access for all previous purchasers will remain.
https://www.blog.poliigon.com/blog/2024-texture-standards
I don't know of any other 3rd party sites actively starting to support Material X, not to mention refreshing their texture libraries from last gen textures. It looks like a first batch of updated materials on their site supports Material X download, for DCCs actively supporting Material X integration.