On Ubuntu I have updated to CUPS 2.2.12 for Eoan (19.10) and found out that the PPD file HP-Color_LaserJet_CM3530_MFP-PDF.ppd of the sample PPDs of cups-filters is not accepted by CUPS any more (also fails cupstestppd) due to the fact that one of the choices of the option "stapleoption" was named "custom".
This case was no big problem as as upstream maintainer of cups-filters I could simply change it to "customsize" (included in the 1.25.3 release of cups-filters) but there seem to be many other PPD files out there in the wild which have an option choice named "custom". For example there are PPD files in HPLIP with this problem, as shown in Debian bug #935918.
Why is "custom" not allowed any more as option choice name from 2.2.12 on? For supporting custom options of CUPS only "custom." is needed, not the simple "custom". Also 2.2.12 is a micro release, it should not have such kind of change.