IMMUTABLE and PARALLEL SAFE function markings

CREATE FUNCTION is_not_distinct(a anyelement, b anyelement)

returns boolean

language sql as $$

  select a is not distinct from b;

$$

IMMUTABLE;

CREATE OPERATOR === (

  LEFTARG = anyelement,

  RIGHTARG = anyelement,

  PROCEDURE = is_not_distinct,

  NEGATOR = !==

);

I have observed that the resulting queries were executed without using parallelisation.

I have learned by asking on Freenode that the reason my queries are not using parallelisation is because I have not configured PARALLEL SAFE.

I find it counter-intuitive that a function with IMMUTABLE marking would require an explicit PARALLEL SAFE. It would seem logical that in all cases where a function is appropriately market as IMMUTABLE it would also be PARALLEL SAFE.

I was wondering what is the reason IMMUTABLE functions are not by default PARALLEL SAFE and if the default behaviour could be changed to make IMMUTABLE functions PARALLEL SAFE?

Thank you,

Gajus