You were always able to manually download higher quality voices, and you still can. It's just moved to the (somewhat hidden) Accessibility preferences now.
If you want to force the download, in the Accessibility pane of System Preferences, select Speech and then under System Voice choose Customise. There should be a checkbox already ticked that says "Upgrade to enhanced quality". Pressing OK will then force the download. If you don't see the checkbox it means it's already upgraded or there's no higher quality version for the selected voice in your language.
In my case it was a 413 MB download. Apple obviously can't bundle all the high quality voices with macOS or it would massively blow out the installer size. Most people only use one voice so it makes sense to make it a post-install download to get the high-quality version.
If this fixes a low-quality Siri voice on new install for you let us know. I'm not certain if this is the fix or whether Apple has a different "medium quality" voice but I figured downloading the high-quality version would do the trick.