There is no Apple's IP address with Private Relay. Apple is using 3rd part companies as its exit nodes to avoid this "whitelist apple ip addresses" concern.

The "Get ready for iCloud Private Relay" session[1] from this year's WWDC makes it seem like there will be a publicly available list of IP addresses used by Private Relay:

Private Relay guarantees that users can't use the system to pretend to be from a different region, so you can continue to enforce region-based access restrictions. Details about the proxy IP addresses will be available as an article associated with this session.

Though I haven't been able to find the aforementioned article.

[1]: https://developer.apple.com/videos/play/wwdc2021/10096/

Here's the article:

https://developer.apple.com/support/prepare-your-network-for...

And their IPs, as mentioned in that article:

https://mask-api.icloud.com/egress-ip-ranges.csv

Seems to be mostly Fastly IP addresses right now, but I'm sure that'll change over time.

I'm running the list using IP2Location and it is showing 3 CDNs now.

(a) Fastly (b) CloudFlare (c) Akamai

I used to work on anti-hijacking "risk based analysis" at Google. Here's what's even more likely to happen: Apple's IP addresses get marked as VPN/Tor ranges and treated exactly the same way as other such ranges. This may trigger e.g. requirements to use two factor authentication on an account if you wish to log in from such ranges.