Thanks for the patch, applied!
You are correct the NEWS file states that this was added in 1.9.0

I checked the code and your patch looks right. I am going to apply it.

Committed.

Thanks. Fix goes into 1.37.

See also D475.

Nope

Here are the patches without any new commands:

@werner Only patches 2 and 3 introduce new commands. What do you think about the other changes?

In case I not already mentioned it: There won't be any new commands to delete a key. Of course you are free to change GnuPG as you like but I won't apply them here.

@werner Thank you for resolving this issue.

See the man page on how to delete subkeys or just the primary secret key with --delete-key.

Actually I have a different approach to fix this bug(let). Please give me a few days.

@werner Thanks for merging the --dry-run patch in 110a4550179f !

In master, I pushed a change, closing.

For future, it would make sense applying your patch, but I wonder if it works on macOS.
Let me check.

It appears this issue was first identified and triaged in 2016: T2879
The subkey deletion feature also showed up in other issues since then:

Without -no-undefined, libtool refuses to create the shared library (dll) on Cygwin, because libtool knows that creating a shared library (dll) on Cygwin does require all symbols to be defined.
Unfortunately but traditionally, by default libtool has to assume a library being created will have undefined symbols.
Hence, if the library to be created is designed to have all symbols defined, libtool needs to be informed about this fact using the -no-undefined flag.
This flag does allow libtool to create a shared library even on platforms known to require all symbols to be defined for shared libraries.

Please explain in more detail what the problem with Cygwin is.