Note that SQL-standard bodies are supported as of 14 [1]:

    Allow SQL-language functions and procedures to use SQL-standard function bodies (Peter Eisentraut)

    Previously only string-literal function bodies were supported. When writing a function or procedure in SQL-standard syntax, the body is parsed immediately and stored as a parse tree. This allows better tracking of function dependencies, and can have security benefits.

[1]: https://www.postgresql.org/docs/14/release-14.html