We've had Digital Cable for about 12 years now and plenty of channels at some point did carry some form of Analogue teletext, I think the odd BBC channel had Ceefax too,
I have ex-Telewest Virgin cable and until very recently I had a full teletext service on the 5 main channels. I assume they took the data from the analogue off-air and then added it to the digital version at the head end.
There was a limited Teletext service transmitted on OnDigital. It was used for switching purposes and transmitted on the Carlton Cinema LCN. It was freely accessable to anyone with a teletext receiver connected via RF to the OnD box on which CC was tuned.
Wasn't it something to do with ONnet, the idea being that broadcasters could add links to their programmes or adverts that you could then click through onto the internet?
It was only possible because the ONdigital boxes had a teletext encoder in them.... as do Sky and cable boxes. As far as I know, no 'Freeview' boxes have them so even if it was broadcast no-one would see it