I put Cyanogenmod on a device and tethered for years without any carrier warnings. Then I accidentally got a device with a locked bootloader and tethered by rooting it to switch a flag; I got a warning from that one. Then I got another open device, put LineageOS on it and tethered uneventfully for another couple years.
I'm sure my custom ROMs played some games to disguise the traffic, but at least on AT&T it seemed to work without a hitch.
I'm sure my custom ROMs played some games to disguise the traffic, but at least on AT&T it seemed to work without a hitch.