Unexpected Redshift support for the ISNULL() function. Is this documented?

**As far as I know, the ISNULL() function is only compatible with T-SQL.** Yet, I am working on a Redshift cluster, using Datagrip, that accepts the ISNULL() function and returns results without error. I'm even getting the contextual helptext on it in the IDE. I have found zero Redshift documentation and zero PostgreSQL documentation that even refers to the ISNULL() function, let alone confirms support of it. **Is there documentation out there for support of ISNULL()? Is this function unofficially supported, since it works, but I can't find any documentation?** Redshift environment info: DBMS: Redshift (ver. 8.0.2) Driver: Redshift JDBC Driver (ver. 2.0.0.3, JDBC4.2) Effective version: Amazon Redshift (ver. 1.0.35480)