It was my understanding that as of JDK12 the old experimental flag -XX:+UseCGroupMemoryLimitForHeap was removed but going to be baseline e.g. all JDK12 apps would read cgroup memory and set that as -Xms and -Xmx internally by default. However when I ran -XX:+PrintFlagsFinal with no -Xmx specified it showed I had only 512MB of memory for MaxHeap in a container with a limit of 2GB of memory allocated.
Did I miss something and the community decided to remove the -XX:+UseCGroupMemoryLimitForHeap flag with no intention of supporting behavior?
This is correct. Java 12 respects cgroup RAM limit (2GB) and ignores the total RAM available on the host (which is larger than 2GB). Since -Xmx wasn't specified the default JVM behavior is to use 25% of available RAM and you end up with 512 MB heap. In practice -Xmx is a specific value which overrides -XX:+UseCGroupMemoryLimitForHeap and other flags governing dynamic heap sizing.
Assuming you don't want to specify -Xmx directly you most likely want to use -XX:MaxRAMFraction=1 which will make the JVM use 100% of the available RAM. Do note that under load it may kill the container.