> you absolutely were required to pay $99 per year for as long as I can remember
You have no idea what you're talking about. I've been a professional Mac developer for 17 years, so I do know what I'm talking about.
The Apple Developer Program has existed for decades, and in fact it used to be way more expensive, i.e., $500 or $3500 depending on level: https://web.archive.org/web/20080513215048/http://developer....
However, membership in the Apple Developer Program wasn't required to distribute Mac software, because Xcode was included with Mac OS X, and Mac OS X did not require apps to be code signed. The code signing requirement started in 2012. You can see the membership benefits in the above Wayback Machine link.
> The perks included receiving a free copy of OSX - which was funny considering you still needed a Mac to work on.
You are right about that, but remember that Mac OS X used to be distributed on disc, and you had to go to the store and pay for it, so having a copy of the latest update mailed to you would have been very convenient.