A completely OOB management network is an amazingly high cost when you have presence all over the world. I don't think anybody has gone to the length to double up on dark fiber and OTN gear just for management traffic.
Hmm... With 5G each blade in the rack could get its own modem and sim card for OOB management.
Why would you do that when you could have 1 sim in a 96 port terminal server?
That's less of an issue. Issue is in how you classify traffic on a network. e.g. Gmail, it's helpful to incident response, should it be used for OOB management.