Index Only Scans with functions

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/18/indexes-index-only-scans.html
Description:

On the documentation I read:

"However, PostgreSQL's planner is currently not very smart about such cases.
It considers a query to be potentially executable by index-only scan only
when all columns needed by the query are available from the index. In this
example, x is not needed except in the context f(x), but the planner does
not notice that and concludes that an index-only scan is not possible."

I tried to reproduce the behavior, but it used the index (which is good):
select version();
drop table if exists t;
drop function if exists add_ten;
create or replace function add_ten(a integer)
returns integer
language plpgsql
immutable
as $$
begin
return a + 10;
end;
$$;
create table t as select generate_series(1, 1000000) a;
create index t_i1 on t (add_ten(a));
analyze t;
explain analyze select add_ten(a) from t where add_ten(a) < 12;

Output:
version
-------------------------------------------------------------------------------------------------------------------------------------
PostgreSQL 18.4 (Ubuntu 18.4-1.pgdg24.04+1) on x86_64-pc-linux-gnu,
compiled by gcc (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0, 64-bit
(1 row)

DROP TABLE
DROP FUNCTION
CREATE FUNCTION
SELECT 1000000
CREATE INDEX
ANALYZE
QUERY PLAN
-----------------------------------------------------------------------------------------------------------
Index Scan using t_i1 on t (cost=0.42..8.69 rows=1 width=4) (actual
time=0.034..0.035 rows=1.00 loops=1)
Index Cond: (add_ten(a) < 12)
Index Searches: 1
Buffers: shared hit=4
Planning:
Buffers: shared hit=8 read=5
Planning Time: 0.154 ms
Execution Time: 0.051 ms
(8 rows)

mydb=#

I also tried some older versions on dbfiddle.uk without success. What did I
do wrong?

Jochen