When cp was invented, 'most filesystems' did not have the first clue about copy on write.

However, I should note that possibly the first company to invent what you describe was Microsoft.

Novell Netware 386 had an NCOPY command which invoked a Netware extension to the DOS API that told the server to perform the entire copy on the server.

But even earlier, OS/2 1.x had a proper DosCopy() system call. Since it could be passed down to the installable filesystem drivers for intra-volume copies, something like the Netware client for OS/2 could in theory turn it into the same protocol call that did server-side copies. There was a NET COPY command in LAN Manager (and LAN Server, if memory serves) that did the same optimization.

* https://www.edm2.com/index.php/DosCopy_(OS/2_1.x)

* https://www.edm2.com/index.php/FS_COPY

> When cp was invented, 'most filesystems' did not have the first clue about copy on write.

Eh, when fork was invented, most (virtual) memory systems did not have the first clue about copy-on-write either. And honestly, it's really not that difficult to support — it's essentially hard links, just with slightly different semantics.