Soulver 3 requires macOS 10.14.4 or later. If you're technically inclined, you might note that this is the first version of macOS that had Swift built right into the system. Soulver is written in Swift, and we didn't particularly want to ship Swift's runtime inside Soulver's bundle.
If you're not running Mojave or later yet, you can still get Soulver 2 (which supports macOS 10.10 or later) on the Mac App Store.
No, it's a separate purchase.
Yes, but due to Apple's limitations, only if you purchase through Safari using this link.
Not this time. Note that Soulver 3 is not a subscription and historically our users have done quite well in terms of updates: Soulver 1 had five years of free updates and Soulver 2 had nine years of free updates.
Soulver 3 for Mac cannot sync with Soulver 2 for iOS, as they use different file formats and math engines. Personally we continue to use Soulver 2 for Mac (for when we need syncing with iOS) alongside Soulver 3 on the same Mac.
No. Soulver uses simple and natural human syntaxes to invoke helpful functions. You can see what particular syntaxes are supported in the syntax reference. It does not use AI/machine learning to analyze your text.