Adobe killed the product activation servers for CS2 products in 2013, and for CS3 products in 2017. For CS2, they offered Activation-free replacement installers and generic serial numbers on their support page. This resulted in a bunch of press[0] about it being a /!\ omg completely free Creative Suite /!\, so for CS3's 2017 shutdown[1] they made you register your original serials to your Adobe Account in exchange for an individualized offline serial and the offline installer[2].
I don't know exactly when, but some time around the end of 2019 or the beginning of 2020 they ended[3] the offline installer program for CS3, removed the ability to generate offline serials in my Adobe Account page or even re-access the offline downloads for my already-generated serials, and have seemingly scrubbed their Knowledge Base of any mention that they ever existed.
I am so thankful I got them while I was able to since aside from needing to tweak the high-DPI handling[4] the CS3 apps work beautifully on my Windows 10 x64 machine. All CS3 applications are only 32-bit, but they're also the final versions with traditional UIs before they gave everything a shiny new Flash-based UI in CS4, so I'm fine with it :)
[0]: https://nofilmschool.com/2013/01/adobe-releases-creative-sui...
[1]: https://web.archive.org/web/20190401002548/https://helpx.ado...
[2]: https://i.imgur.com/ae1SWN5.png (my screenshot!)
[3]: https://community.adobe.com/t5/photoshop/locked-photoshop-cs...