It doesn't work (it could, but hasn't been implemented).
Back-patch to 12, where shared_memory_type arrived.
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/163271880203.22789.1125998876173795966@wrigleys.postgresql.org