I can confirm this issue.
Searching for [mM][aA][nN].{0,3}[pP][aA][gG][eE] using Visual Studio Code (which uses ripgrep under the hood) yields 260 results.
The vast majority seem to use the first spelling, that is "man pages".

So now to find different ones, we tweak the regex to not search for a space in the middle [mM][aA][nN][^ ]{0,3}[pP][aA][gG][eE]. At this point, a lot of results will be things like variables or links.
e.g.
Variables: generate_manpage, ${MANPAGE}, MAN_PAGE_LOCATION, runManPage, ...
Links: https://www.libelektra.org/manpages/kdb, https://en.wikipedia.org/wiki/Man_page, ...

Proposal
I'd propose unifying the spelling to "man page" with a separator. Although, depending on the context, a different separator can make sense. generate_manpage would thus become generate_man_page. Meanwhile something like runManPage is already fine under that proposal.

Another thing that we should take a look at is file names. There's at least one file containing the words "man page", namely ElektraManPage.cmake.

And finally, the tricky part that I'm unsure about:
How does one go about updating the https://www.libelektra.org/manpages/... links? According to the Google guidelines we could use https://www.libelektra.org/man-pages/.... At the same time, we do want to have a redirect from our old https://www.libelektra.org/manpages/... URLs to the new ones, after all, a good URL should never break.

Most English sources I found in a quick Google search use "man page". Both "manpage" and "man-page" seem to be more of a German thing...

Most English sources I found in a quick Google search use "man page". Both "manpage" and "man-page" seem to be more of a German thing...

Your research result squares with the proposal from @stefnotch:

I'd propose unifying the spelling to "man page" with a separator.

So normally, the separator is a space, but in many cases, spaces are not allowed, so either a CamelCase variant manPage, or other separators like - or _ must be used.

The file name ElektraManPage.cmake looks fine to me.
The man pages on the website refer to files on the GitHub respository available at https://github.com/ElektraInitiative/libelektra/tree/master/doc/help.
But you're right, if we want to change the URLs to contain man-pages instead of manpages, we should create a redirect or offer the man pages under both URLs.

Nice triage, thank you!

We'll take a stab at this for T1

Categories of different spellings
bad:
Manpage
manpage
MANPAGE
manpages

good:
man_page
MAN_PAGE
manPage
Man_page

good except in some cases:
man-page (fine in URLs or file names, bad in plaintext)

More bad results can easily be found with [mM][aA][nN][^-_ ]{0,3}[pP][aA][gG][eE]

Man_page

Is this for file names? Otherwise you better would say "Man page" (without quotes)?

For file names or other cases where underscore_case is the way to go.
e.g. generate_manpage would become generate_man_page