VES - a VA contractor - just called me to schedule a "routine" C&P exam. Is this a VA scheme to lower vets' disability ratings?

Sorry, it’s not a scam. If you are not P&T the VA will forecast routine future C&P appointments. Take a look at your last rating decision and I’d venture a guess that it said you have a routine C&P in April 2020.
Having said that contact your VSO for help in preparing for the exam. They can help you determine if you should request an increase in your Zero Percent service connected disabilities or and others you may have.
I’m guessing that you have all your medical records from the time you separated going forward. Provide them to the VSO or upload them yourself via VA.gov. If you’ve been treated at the VA they should already have them. Having said that I’d make sure I took my records showing continued treatment for service connected disabilities. That way you can prove your conditions didn’t miraculously get better.
Just make sure you don’t miss your exam!

It's an exam to re-evaluate your injury/illness/issue. Is it a scam? No. Could your percentage go down? Yes, it could go down if there is not documentation to support the claim or prove it is ongoing. Could your percentage go up? Absolutely. If you have history to support the issue has gotten worse they would increase your percentage. I just had a series of evals when VES in February and my percentage was bumped from 70% to 90%.

Listen, it is a way to ensure that the VA is not being scammed. They will periodically review your status, and test for what is current and active. They will do this until you reach a certain age or until you are found to be 100% and permanently disabled. I am sure your conditions are legitimate, so you should have no fear. Since the VA is being ripped by people you should understand the necessity, many conditions improve.