IndexAmRoutine aminsertcleanup function can be NULL?

Hi hackers.

I happened to notice that many contrib module indexes do not implement
an aminsertcleanup function:
e.g. amroutine->aminsertcleanup = NULL;

But, unlike many of the other interface functions, the aminsertcleanup
does not say "/* can be NULL */" in the typedef struct IndexAmRoutine
[1].

What's going on there? Is it just an accidentally missing "/* can be
NULL */" comment?

Same also in the documentation... [2]

======
[1] https://github.com/postgres/postgres/blob/master/src/include/access/amapi.h
[2] https://www.postgresql.org/docs/devel/index-api.html

Kind Regards,
Peter Smith.
Fujitsu Australia