Just for transparency, I spent some time on the forums tonight while I was imaging a machine and ran across this thread. I attempted to reproduce this, but I'm not able to.
This is Firefox 81 on Win 10 from my home network:
https://documentcloud.adobe.com/link/track?uri=urn:aaid:scds:US:53f6ae60-41c3-44d9-9063-0ada598a5148
I did notice that when I checked on Firefox for Mac I was not presented with an opt-out dialog, but that's because there's not a bundled offer targeting that browser and OS combination in the US. In that instance, the product download center provided the full 21MB installer without a bundled offer payload.
As you've observed, Adobe monetizes Flash Player downloads through various bundled offers delivered via the Adobe Product Download Center. The distribution happens downstream (I work on the actual product), and my knowledge of how that system works is fairly high level. The gist is that the downloads are typically monetized through some kind of product offer. Those offers vary depending on the operating system, browser, language and geographical location of the user.
I'm always more than happy to take a look at stuff like this, and to advocate for users where I can. Also, to be clear, I'm not disputing that this happened. At the same time, I can't have an actionable conversation with another engineering team without the kind of high quality forensic detail that would allow them to conduct an effective investigation.
The easiest way would be to reproduce it, but given the complexity of that targeting ecosystem, that's pretty hard. Where I can't see the issue, if I had something like a screenshot, are pretty precise time (e.g. the file creation timestamp from your end) that the problem happened, your public IP, the operating system and browser (and versions), and the name and/or hash of the file that was downloaded, then I could realistically send someone to go digging through the literally billions of records in Splunk to determine a root cause and get it fixed.
If you run into this again, please absolutely feel free to send me a DM. I'll do my best to escalate it internally and get it fixed. I don't have any input on our business approach, but I'm invested and empowered to make sure that those systems perform to spec. I'm more than happy to continue to advocate on this front when I have enough info to be effective.