This is an interesting post. The crossdomain.xml file itself is a precursor to CORS in modern browsers. It's just a little file that tells Flash Player "yeah, the administrator that owns this server says that it's cool if you load content from here". It's not actually doing anything for your application.
What I would expect though, is that your application is requesting or including other stuff from https://fpdownload.adobe.com/pub/swz/... like some of the signed, pre-compiled libraries for the Flex framework or TLF text components (.swz files).
I'm guessing that the actual assets that matter are already pre-cached on your system (this is the point of the signed, pre-compiled libraries -- you can download them once and cache them -- and they work across all websites where those libraries get used --- vs. downloading them every time you load the page). If you repeated this exercise in private browsing mode, you'd probably see requests for the actual dependencies. It may require going to Control Panel > Flash Player > Advanced > Delete All though. It's been several years since I've personally tested this feature.
What I can confidently say is that we're aware that these files exist and don't have any immediate plans to delete them. That said, Flex was given to the Apache Foundation in 2011, and we're still hosting those files in 2020. There will be a finite end date at some point, but it won't be anytime soon.
We're just starting to talk about what the long-term disposition of those files is, and I expect that we'll have public communication about it once we've nailed down a detailed plan.
That said, I believe that in the event that those files are not accessible or cached (like if you set fpdownload.adobe.com to 127.0.0.1 in your hosts file), Flash Player will fall back to looking for those .swz files on the domain that you loaded the parent SWF from. If you clear the cached .swz files using one of the methods above and request a page with fpdownload unavailable, you should be able to see where Flash Player is expecting to find those files on your domain.
I'll try and find some time to test this on my machine next week and get a definitive answer as to where the .swz files live respective to the parent SWF that gets loaded. When they were introduced, it was really a loading-time optimization for Flex applications, and given that Flex has been EOL for several years, I'm having a hard time finding good authoritative documentation on the fallback behavior.
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