> I believe this was implemented in issue33234

I don't think so. The bytecode in Python 3.9 still uses "BUILD_LIST 0":

Python 3.9.0a4+ (heads/daemon_thread_runtime2-dirty:48652767d5, Mar  7 2020, 00:56:07) 
>>> def f():
...     for i in range(10000):
...         [j for j in range(10000)]
... 
>>> import dis; dis.dis(f)
(...)

Disassembly of <code object <listcomp> at 0x7ffab2c9fd40, file "<stdin>", line 3>:
  3           0 BUILD_LIST               0
              2 LOAD_FAST                0 (.0)
        >>    4 FOR_ITER                 8 (to 14)
              6 STORE_FAST               1 (j)
              8 LOAD_FAST                1 (j)
             10 LIST_APPEND              2
             12 JUMP_ABSOLUTE            4
        >>   14 RETURN_VALUE