I hadn't noticed earlier, but CockroachDB 22.1 has introduced ON CONFLICT .. ON CONSTRAINT support:

• sql: support INSERT ... ON CONFLICT ON CONSTRAINT cockroachdb/cockroach#28161
• sql: implement ON CONFLICT ON CONSTRAINT cockroachdb/cockroach#73460

We're currently emulating it if the constraint meta data is available, i.e. if we can replace it by a column list.