Namecheap - WhoisGuard Privacy Protection is now free – forever!
 

https://www.namecheap.com/security/whoisguard.aspx

When you register an eligible new domain name or transfer an existing eligible domain to Namecheap, you'll receive WhoisGuard privacy protection absolutely FREE for as long as you keep your domain with Namecheap.

Presumably that means it doesn't apply to existing domains in an account.

It does apply to existing registrations.
The side panel at https://www.namecheap.com/security/whoisguard.aspx says

Name Silo has been offering it for years...

I think most will be offering free WHOIS due to GDPR coming in. Reduces the chances of someone's personal information being revealed.

Maybe this is my anxiety disorder playing tricks on me again, but... if the registrant in the WHOIS database is the actual owner of the domain, then doesn't WHOIS Privacy lead to the registrar owning the domain (by masking the customer's data and replacing it with own data) instead of the registrant/customer owning the domain?

For this reason I never chose to activate private WHOIS.

I did see, last time I checked my own domains in who.is, whois.sc and whois.com, that most of the information was no longer visable. It seemed like a temporary outage of the WHOIS database ; maybe it's just the registrar no longer displaying the registrant's data in the WHOIS due to new privacy laws?

Not really a fan of GoDaddy, but I think their explanation is pretty good: I think there have been some cases where having the WHOIS information private caused some issues when a domain registrar went out of business and the rightful owners of the domains were left to prove they actually did own the domains. Not sure if it is a big problem or not.