READ COMMITTED isolevel is implemented and is default now.

src/backend/access/heap/heapam.c

src/backend/access/nbtree/nbtinsert.c

src/backend/access/transam/xact.c

src/backend/commands/trigger.c

src/backend/executor/execMain.c

src/backend/executor/nodeIndexscan.c

src/backend/executor/nodeSeqscan.c

src/backend/nodes/copyfuncs.c

src/backend/parser/gram.c

src/backend/utils/time/tqual.c

src/include/nodes/execnodes.h

src/include/utils/tqual.h