Apple says the VP9 codec no longer works on Catalina. I want this codec for my older OS. Where can I find it?

Apple has a page listing "Media formats affected by the transition to 64-bit technology":

In macOS versions up to and including macOS Mojave, third-party software has extended the QuickTime 7 framework to support many incompatible media formats. In macOS Catalina, the QuickTime 7 framework will no longer be available, so incompatible formats won't be supported in Final Cut Pro, Motion and Compressor.

Here are examples of media formats affected by this transition:

The second item on the list of affected formats reads:

AV1 / VP9

I am on an older version of macOS, and I would very much like to have native support for VP9 in QuickTime! Unfortunately, I am not aware of any QuickTime components that add compatibility with VP9. Perian, which is listed seperately, only ever supported up to VP8.

Was there ever a VP9 QuickTime Component, and if not, what is Apple's page referring to? I am looking for the name of the specific software being referenced.

Welp, I'm still not sure whether there ever was a VP9 component, but there is now because I made one. I managed to update FFusion (the core component of Perian) to work with a newer version of libavcodec.

https://github.com/Wowfunhappy/FFusion

At long last, I can watch 4K Youtube videos in QuickTime Player 10.2. 😎

Was there ever a VP9 codec, and if not, what is Apple's page referring to?

No, there is no Quicktime Player plugin that adds VP9 codec support to it (as far as I know). Apple is referring to codecs built on Quicktime Framework.

Quicktime Framework is a multimedia library provided on macOS.

The Quicktime Player app, Quicktime Broadcaster, QuickTime Streaming Server etc. all use the Quicktime framework to play videos and / or manipulate them (edit or re-encode them).

Many video editors on macOS use the Quicktime Framework too, to add support for codecs (like VP9) for use in these video editor apps.

(Note: Codec support added to an app doesn't make it available to ALL apps that use the framework - so for example, Quicktime Player will not be able to automatically use VP9 codec added to another app just because it is using the Quicktime Framework).

With macOS Catalina, Apple has now replaced the old QuickTime 7 Framework with a new Quicktime X framework. Due to this big change, any codec built on the old Quicktime 7 Framework will no longer work on macOS Catalina.

More info here: Apple is killing QuickTime 7 in macOS 10.15.

(However, this doesn't mean that VP9 (or other codecs) cannot be used to watch VP9 / webM videos on macOS Catalina. You can still watch VP9 encoded videos using other media players like MPV, VLC or IINA on macOS Catalina. You can also create / encode VP9 media with Handbrake on macOS Catalina.)