[mackmgg]

I'm not sure why Notes needs to be bundled with the OS update, but Safari isn't. You can download Safari 17 for macOS Ventura as well! Only the features that require integration with other apps (such as auto-filling 2FA codes from Mail) require Sonoma.

[philistine]

The fact that Safari is available for multiple OS versions might be a remnant of the work done to ship Safari for Windows.

[Affric]

I reckon it's more likely that Safari as a browser represents a more important set of security holes and therefore Apple feels greater obligation to patch patch problems (and that's easier to do by simply shipping the whole app rather than patches for old versions).

[bombcar]

This is exactly it - as long as they support macOS for X years they need to release safari updates for that long, so they might as well decouple that one.