Inside mod-extra.list there is the list of modules that should be put inside kernel-modules-extras.
The problem is that, if a module (present in mod-extra.list) is required by another module (not present in mod-extra.list) the module is not added inside kernel-modules-extras, but it'll be inside kernel-modules and NO warning is printed or reported.
For example sctp.ko and sctp_probe.ko was added to the list. Now sctp_probe.ko doesn't exists anymore (since kernel 4.16), but sctp_diag.ko was added and so scp.ko is inside kernel-modules, instead of kernel-modules-extra.
We could abort the kernel build when a module is removed from mod-extra.list so the maintainer can remove it from the list, if the module is needed inside kernel-modules, or the maintainer can add the reverse dependencies inside the list, if the module is needed inside kernel-modules-extra.
Or, as alternative, we can add a warning, but somebody or some tools should check for the warning