So CUPS has decided to standardise on only IPP Everywhere:
This means dropping support for raw printer queues, PPDs and printer specific filters. Using and of them gives these warnings from modern lpadmin:
> lpadmin: Raw queues are deprecated and will stop working in a future version of CUPS.
> lpadmin: Printer drivers are deprecated and will stop working in a future version of CUPS.
For us that means we'll have to drop raw support and adjust how the normal printing works though thinlocal and nearest.
Not sure what this means for backend support or lpd support. We may have to switch architecture for printing on both the server or client.
The timeframe is unclear, but they are suggesting that this is long term so hopefully no rush.