There's a few things to unpack here:
Whether this has to be asked on Meta Stack Overflow
This isn't necessary. Stack Exchange reviews all the child meta discussion sites: you are fine asking questions about stuff that happens on Drupal Answers, even if what happens here also affects other sites.
Meta Drupal Answers shouldn't just be the constant retag fest it currently is, and every user should feel free to discuss things related to Drupal Answers, whether it only affects this site or it affects multiple sites.
Using 403 Forbidden over 404 File Not Found
This is a common misconception of HTTP 4xx status codes: in this situation,
403 Forbidden is always incorrect, and
404 File Not Found is actually the closest error to reality.
I explained a bit more about the difference between 403 and 404 in an answer on Programmers, but in short,
403 Forbidden is the server saying "I understand the request, but I will not handle it now or in the future, so stop trying" whereas
404 File Not Found is "this page isn't available to you now, but it may be available in the future."
Preventing discovery of links not accessible to a user
The purpose of the privileges page is discovery: you want people to see what extra reputation gives them and what they can do when they hit certain milestones. You're going to have to talk about things that users cannot access, and provide links to that stuff for the people who can.
Now, they could write two separate pages: one for when you have the reputation and one for when you're aspiring to get to that reputation, but that's a maintainability nightmare.
The other option is to disable the links for those who don't have access, but this goes against one of the main design principles of the network: to not show disabled links. Joel Spolsky, one of the founders of Stack Overflow and Stack Exchange, once wrote:
A long time ago, it became fashionable, even recommended, to disable menu items when they could not be used.
Don't do this. Users see the disabled menu item that they want to click on, and are left entirely without a clue of what they are supposed to do to get the menu item to work.
Instead, leave the menu item enabled. If there's some reason you can't complete the action, the menu item can display a message telling the user why.
So you're left with leaving the links there and throwing an error when a user who can't access it does so. Which brings us to your other suggestion...
Provide a better error message for users who hit those pages
This I'm 100% with you on, and there's definitely precedent that Stack Exchange would do it:
Perhaps something like:
This page is not available for your reputation. Please see the privileges page to see how much reputation you need to unlock it.