@JohnFlower You totally avoided the 2nd part of that statement. Get on the phone and talk to them then... There is no point complaining until you've done this.
I work in an industry where breaking changes are common to customers - I don't go and tell the customers still on Windows XP that I am turning off TLS 1.0 and TLS 1.1 on the apps I support as an example. But you've got to look at this the other way. Somebody non-technical who just has their Fritz!Box doing their home phone and internet and uses WiFi won't care about the CG-NAT change. If you tell them, you potentially cause panic as customers may misunderstand what it means to them. For almost all of their customers this is not a breaking change as most IoT doesn't require a port forward (and if it does - it should be in the bin), gaming will still work on most platforms and in the rare cases something does fully break then they'll make it right for the customer again.
As also stated - this exact move was done by other ISP's in NZ who didn't tell customers. Telling the customer just isn't the smartest thing to do in this case.