I believe this is an intentional feature in recent Texinfo versions.
To get the warnings back, you need to run makeinfo with the
command-line option "-c CHECK_NORMAL_MENU_STRUCTURE=1".
Thanks for the hint. I reported a similar thing in July 2023,
https://lists.gnu.org/archive/html/help-texinfo/2023-07/msg00004.html
and my understanding of Patrice's reply is that the config setting
was no longer intended to be needed in 7.1:
https://lists.gnu.org/archive/html/help-texinfo/2023-07/msg00005.html
And in fact my example document from 2023 does get a warning in 7.1.
So that is good.
My new report is about a similar, but not identical, case which
still does not get a warning in 7.1. I presume(d) it should (i.e., a bug).
Thanks,
Karl
