Postgres 15 has added an indnullsnotdistinct boolean field to the pg_catalog.pg_index table:

indnullsnotdistinct bool
This value is only used for unique indexes. If false, this unique index will consider null values distinct (so the index can contain multiple null values in a column, the default PostgreSQL behavior). If it is true, it will consider null values to be equal (so the index can only contain one null value in a column).

We may want to support this too for compatibility.

Jira issue: CRDB-21875

Epic: CRDB-23454