All old Zotero versions are available using the same URL schema -- you can just check the URL schema for 9.0 for your OS and use that for 8.0.5 That said, it's Cita not supporting Zotero 9, not the other way around, and it's in all likelihood, a trivial update for the developer.
Instead of downgrading Zotero, have looked at using one of the Cita betas? I understand the latest stable release (version 0.5.5) is old, but if you actually check on the release page, as of version 1.0.0-beta.12 Cita should support Zotero better. Which version do you have installed currently? https://github.com/zotero-cita/zotero-cita/releases
The latest Cita beta 1.0.0-beta.18 still shows v8 as the max compatible Zotero version (open XPI file as ZIP, then open the manifest.json text file inside the ZIP to check that value).
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That said, it's Cita not supporting Zotero 9, not the other way around, and it's in all likelihood, a trivial update for the developer.
I understand the latest stable release (version 0.5.5) is old, but if you actually check on the release page, as of version 1.0.0-beta.12 Cita should support Zotero better.
Which version do you have installed currently?
https://github.com/zotero-cita/zotero-cita/releases
So an issue should be raised with the developer to update compatibility:
https://github.com/zotero-cita/zotero-cita/issues