10.9 was the first "free" macOS release, prior to that, it was a paid upgrade.
"Mac OS X 10.1 was released on September 29th, 2001, but that date depends on your definition of "released." Update CDs were handed out for free after Apple's keynote speech at the Seybold publishing conference in San Francisco. People who did not attend Seybold have several ways to get the update. Starting on the 25th, free update CDs are being handed out at Apple stores and some other retail outlets. This free update does not include the developer tools, but those can be downloaded by all ADC online members (free registration required) from Apple's web site. Finally, owners of Mac OS X 10.0 can order a full update containing a Mac OS 9.2.1 CD, a Mac OS X 10.1 update CD, and a developer tools CD through Apple's Mac OS X Up-to-Date program at a cost of $19.95 (plus state tax). The 10.1 update is not available for free download."
IIRC the next paid upgrade 10.2 was the more typical for the period 129 dollars. I was in a line of three people at my local Apple store for the free disc back in 2001.