create database test;
\c test
create extension jsquery;
create table test(j jsonb);
insert into test values ('{"id":1,"test":"test"}');

CREATE OR REPLACE FUNCTION has_test(j jsonb) RETURNS boolean
LANGUAGE plpgsql IMMUTABLE
AS $$
DECLARE
ret boolean;
BEGIN
SELECT $1 @@ '$.test = test' into ret;
return ret;
END;
$$;

CREATE INDEX test_index_plpgsql -- does't work in vacuum
ON test
USING btree
(has_test(j));

CREATE INDEX test_index_clear -- works fine
ON test
((j->>'id'))
WHERE j @@ '$.test = test';

vacuumdb -fzv --dbname=test
vacuumdb: vacuuming database "test"
INFO: vacuuming "public.test"
INFO: "test": found 0 removable, 1 nonremovable row versions in 1 pages
DETAIL: 0 dead row versions cannot be removed yet.
CPU: user: 0.01 s, system: 0.00 s, elapsed: 0.01 s.
vacuumdb: vacuuming of database "test" failed: ERROR: operator does not exist: jsonb @@ unknown
LINE 1: SELECT $1 @@ '$.test = test'
^
HINT: No operator matches the given name and argument types. You might need to add explicit type casts.
QUERY: SELECT $1 @@ '$.test = test'
CONTEXT: PL/pgSQL function public.has_test(jsonb) line 5 at SQL statement

ii postgresql-11 11.0-1.pgdg16.04+2 amd64
Package: postgresql-11
Status: install ok installed
Priority: optional
Section: database
Installed-Size: 43753
Maintainer: Debian PostgreSQL Maintainers [email protected]
Architecture: amd64
Version: 11.0-1.pgdg16.04+2
Provides: postgresql-contrib-11
Homepage: http://www.postgresql.org/
Postgresql-Catversion: 201809051

ii postgresql-11-jsquery 1.1.1-1.pgdg16.04+1 amd64
apt-cache show postgresql-11-jsquery
Package: postgresql-11-jsquery
Source: jsquery
Version: 1.1.1-1.pgdg16.04+1
Architecture: amd64
Maintainer: Debian PostgreSQL Maintainers [email protected]
Installed-Size: 290
Depends: postgresql-11, libc6 (>= 2.14)
Homepage: https://github.com/postgrespro/jsquery
Priority: optional
Section: database
Filename: pool/main/j/jsquery/postgresql-11-jsquery_1.1.1-1.pgdg16.04+1_amd64.deb
Size: 127560
SHA256: b6fdd08d91b83ca5ababe56af3c58dc2891a940102f838313d9f2bbcb8647974
SHA1: 88cab2e08e988ceb1811a82b3aec4c9f6838af44
MD5sum: 586f57fe09dd34c397ee17d2e17d2a95

ii postgresql-common 195.pgdg16.04+1 all

for pg9 - no problem