According to the documentation of Postgres 12 [1] it is possible to use so called non-deterministic collations, which may express case-insensitivity. If that is what you need.
The documentation of Postgres 11 [2] states that this was not possible:
Note that while this system allows creating collations that “ignore case” or “ignore accents” or similar (using the ks key), PostgreSQL does not at the moment allow such collations to act in a truly case- or accent-insensitive manner. Any strings that compare equal according to the collation but are not byte-wise equal will be sorted according to their byte values.
The documentation of Postgres 11 [2] states that this was not possible:
[1]: https://www.postgresql.org/docs/12/collation.html [2]: https://www.postgresql.org/docs/11/collation.html