[temac]

ISDN is an edge point to point protocol (although used to get on a network). I don't really see how it has to do specifically with anything related to AD vs Azure (a data link is a data link, you could has well have used an analog modem, or a custom solution involving an avian carrier), neither do I see how ISDN should be considered as anything else than obsolete technology in all regards nowadays, no matter the nostalgia factor.

BTW B channel is for "Bearer", and D is for "Delta" or maybe "Data". (Wild hypothesis: maybe it was initially for "Data" at a time when B was seldom used for things other than digitized voice possibly with a bit stolen for in-band signaling)

On my side I consider that ATM was pretty sweet at a time when IP&co was complete garbage (and still somewhat is), but I know who won.

As for CICS, well, mainframe are still not dead. I suspect they won't, because why would they? But the model is not the same as the modern "cloud" stuff, at all levels.