[afiori]

My superficial understanding is that arm does not prevent from sharing implementation details of your own design but most chips also license a starting implementation that has such limitations. So the end result is often more restricted than the ISA licence some would require

[duskwuff]

Most ARM licensees aren't permitted to create custom implementations, only to use IP cores provided by ARM. There are a couple of companies who do have an architectural license, allowing them to create their own implementations, but there are only a few of those and they aren't likely to share. (It's also possible that the terms of their license prohibit them from making their designs public.)