Mac OS versions compatibility with iOS versions

The following link indicates the post on Apple Support that. indicates how to downgrade to a lower version of iOS.

https://discussions.apple.com/docs/DOC-8871

The link given below is mentioned in the above post:

https://www.imobie.com/support/how-to-downgrade-from-ios-12-to-ios-11.htm

If I upgrade my Mac would it not become slow as I probably have to upgrade to Catalina from Snow Leopard. Would the newer version of MacOS recognize the iPhone backup as its been done in the very much older Snow Leopard?

[Edited by Moderator]

Repeating my query:

What version of iOS should I downgrade to so that my Mac with OS 10.6.8 will recognize my iPhone which is currently on 12.4.9 and I can restore my backups?

Is there a Mac OS version / iOS version compatibility chart?

Thanks in advance!